After the glorious gains of the ’90s, you can’t fault any mutual fund investor who’s dreaming big dreams for the 2000s too.

The bull market is going full force as the calendar gets set to turn over. Let the good times roll.

Before the whole show begins, though, hear a brief plea to keep your expectations from getting too exuberant. Big gains in the big stock averages have become all too easy to get used to these past five years.

“Investor expectations are inflamed and unrealistic,” says Lawrence Lasser, chief executive of Putnam Investments Inc., which runs a $260 billion fund group. “We’re living in an extraordinary bull market. I hope it keeps up. But this is not the way markets always are.”

If you have any experience with mutual funds, you’ve heard alarms like that before. When the Standard & Poor’s 500 Index soared 37 percent in 1995, fund managers warned investors not to count on another big year in 1996.

After a 23 percent gain that year, the experts said it was time to be doubly cautious. When the index gained 33 percent in 1997, the market was really overextended. By the time it posted a 28 percent rise in 1998, the voices of caution had lost all credibility. This year, the S & P; is up 15 percent with a few weeks to go.

So it’s plain as day that a recent strong showing is no reason, by itself, to pull back from stock investments. But it can be very dangerous to play the momentum game, attempting simply to ride the bull-market wave for as long as it keeps going. Recall 1929.

“The biggest mistake you can make in investing is to abandon a well-conceived strategy because it hasn’t worked very well over the short term, and chase the latest hot trend,” said Boston investment adviser George Putnam III in his newsletter The Turnaround Letter. “The stock market is very cyclical. The hottest segment of the market today may be headed for the deep freeze tomorrow.”

There is a way for investors to deal with these uncertainties. To start with, you set up an investment plan that’s based on what you hope to accomplish rather than a guess at what the markets might do next.

For a teen-ager with paid-up tuition and no financial obligations in sight for years, a portfolio of nothing but aggressive stock funds might make sense whether you think stocks are expensive right now or not. Likewise for a retirement plan you don’t expect to tap for 20 or 30 years.

In most other circumstances, though, you diversify between stock and bond or money-market funds. That way, you allow for both the chance that stocks will keep rising and the possibility that they won’t.

It takes discipline and a belief in what you’re doing to stick with this approach, regularly “rebalancing” out of your biggest winners into your other funds to keep your assets allocated according to your plan.

“I am a compulsive rebalancer, and it is greeted with skepticism by most,” said Peter Moran, a Fairfield, Conn., investor who helps manage the endowment fund at a nearby school. “It will continue to look foolish until the large technology stocks falter something the market is trying to convince us will never happen.”

Nobody can say when the computer and telecommunications stocks now leading the markets higher might give up their starring roles. “But it’s foolish to think that the current madness for tech stocks can continue unabated,” said Eric Kobren, a fund analyst in Massachusetts.

“Remember that high-growth areas attract lots of competition. Early in the century, there were over 50 automakers in the U.S. Most of the personal-computer manufacturers of 20 years ago are long forgotten.”

By the reckoning of Bloomberg analytics, the average stock mutual fund has gained 18 percent a year over the past five years. Success of that sort will make anybody feel smart.

In planning for the 2000s, though, a dash of humility can keep things in perspective. Consider that Warren Buffett, acclaimed as the greatest investor of his age, is down 20 percent this year, using the price of his Berkshire Hathaway Corp.’s stock as a gauge. If he can’t win every round, well, maybe nobody can.

Driving the Market

As yearend approaches, the list of the top-performing mutual funds for the past five years leaves no doubt at all what’s been driving the great bull market.

Electronics, computers, telecommunications. One of those magic words, or the catchall term “technology,” appears in the name of six of the 10 funds with the biggest gains since late 1994, as tracked by Bloomberg analytics. Stocks of such companies figure prominently among the investments of all 10.

Fidelity Investments of Boston, which runs the largest fund group, captured three of the top four spots in the performance rankings. The $4.2 billion Fidelity Select Electronics Fund ranked first with a gain of 52 percent a year; the $2.3 billion Fidelity Select Computers Fund was No. 3, with a 44.9 percent annual return, and the $2.3 billion Fidelity Select Technology Fund was No. 4, up 44.7 percent.

The $1.3 billion Rydex OTC Fund, with a 51.6 percent annual gain, held second place. The fund, set up to parallel the return of the Nasdaq Composite Index, has more than 40 percent of its assets in these five tech stocks: Microsoft Corp., Intel Corp., Cisco Systems Inc., MCI Worldcom Inc. and Dell Computer Corp.

In fifth place stood the $26.2 billion Janus Twenty Fund, which grew so fast it closed to new investors in April. Janus Twenty, so named because it was set up to concentrate on 20 to 30 stocks, has been a big investor in America Online Inc., Microsoft and Cisco.

Here’s a look at performers No. 6 through 10: Invesco Technology (+41.4), Robertson Stephens Emerg Growth (+41.3), John Hancock Global Technology (+40.4), Flag Investors Communications (+40.1), and Delaware Select Growth (+40.1).

Five-year rankings pack a lot more punch than quarterly, annual or even three-year lists. They measure a period most advisers consider the minimum to qualify as long-term, and funds are set up as long-term investing vehicles.

A five-year period also screens out small funds on a hot streak and other flash-in-the-pan performers. Notice that all our Top Five have assets of $1 billion or more.

Notice too that there aren’t any funds specializing in Internet stocks. While Internet funds have run up gains that would put them among or even ahead of the leaders, the first of them wasn’t organized until 1996, and so none has a five-year history yet.

The past five years, as it happens, encompass one of the largest advances in stock market history. Since late 1994, at the end of a sluggish, mixed year for stocks, the Nasdaq Composite has quintupled, soaring at a 38.1 percent annual rate. Meanwhile, the Standard & Poor’s 500 Index has posted a 28.7 percent annual gain.

This electronics-powered boom has steamrollered fund managers and investors who distrusted it, preferring instead to stick with stocks that look good by traditional earnings-focused analysis.

Chet Currier is a columnist for Bloomberg News.