Most stock market investors do best with the long-term approach, buying stocks or stock funds they hope to hold for years.

But you take that approach too far if you fail to address the problems that inevitably crop up from time to time.

Indeed, many 401(k) investors have fallen into a bad habit of counterproductive passivity, according to a new study by Hewitt Associates, the benefits consulting firm based in Lincolnshire, Ill.

Hewitt looked at the accounts of more than 500,000 401(k) participants in 2001, when all the major stock indexes were hammered by the Sept. 11 attacks, only to rebound in the final months of the year.

The good news is that these investors clearly didn’t panic and pull their money out. On the other hand, most seemed to simply disconnect from their accounts, failing to make any adjustments as conditions changed, Hewitt concluded.

For the year, only 19.5 percent of the investors made any change, such as shifting money from a losing fund to a winner, or changing the mix of stocks and bonds. That’s down from 2000, when 30 percent of plan participants made at least one trade in their accounts.

Stock prices, of course, are down again this year. And Hewitt’s continuing surveys indicate investors are maintaining this hands-off approach.

Most disturbing is the finding that 29 percent of the investors surveyed had at least three-quarters of their 401(k) money invested in their own company’s stock. In fact, 15 percent of the participants had all their money in their employer’s stock.

What should 401(k) participants be doing?

First, they should lighten up on company stock. It’s too risky to have both your livelihood and retirement funds tied to the fate of just one company. Generally, a small investor should have no more than 10 percent of her investment portfolio in any single stock.

When Enron Corp. tumbled into bankruptcy last fall, many employees who were heavily invested in the company’s stock were devastated. Apparently, not many investors saw this as the object lesson it was.

Second, 401(k) participants should exploit the special opportunities offered by these tax-favored accounts. Because selling one asset to buy another does not trigger a tax on profits in a 401(k), as it would in an ordinary account, the 401(k) can be the ideal place to make asset-allocation adjustments.

When you set your asset-allocation goals, consider all your accounts together 401(k) and taxable and apply the asset allocation to the whole nest egg. It doesn’t matter if each account strays from the asset allocation breakdown so long as they all meet the goal when taken together.

Suppose your long-term strategy called for keeping 60 percent of your investment assets in stock funds, 30 percent in bond funds and 10 percent in money-market funds.

If falling stock prices caused your stock-fund allocation to drop to 50 percent, you might want to sell some bond-fund shares to obtain money to invest in more stock funds to get back to the 60 percent target.

Selling bond-fund shares from a taxable account would trigger a capital gains tax on any profits. But if you confined the sales to bond-fund shares held in your 401(k), there would be no tax.

Certain types of investments go best in different types of accounts.

Bonds and bond funds go well in 401(k)s (or IRAs) because the tax deferral means you don’t have to pay income tax each year on the interest payments you receive, as you would if you held those income-producing assets in a taxable account.

The same goes for stocks or stock funds that make dividend payments, which are taxed each year as income. (Municipal bonds and funds are an exception. Since their interest payments are tax-free, it’s pointless to keep them in a tax-deferred account.)

On the other hand, stocks and stock funds that don’t pay dividends may serve you best in a taxable account, especially if you plan to hold them for a long time. With these investments, profits come from rising share prices. There’s no annual tax on those gains the tax comes after you sell.

When you sell from a taxable account, the most you’ll pay will be the 20 percent capital gains tax.

But sell the same stocks or stock funds held in a 401(k), and you’ll pay at the income tax rate on any profits realized. For most investors, that rate is 27 percent to 38.6 percent.

So the 401(k) shouldn’t be something you pour money into and forget. You should watch your holdings, dumping the ones that are no longer promising and moving your money to ones that look better. And you should use the 401(k) to make asset-allocation adjustments that won’t trigger ugly tax bills.