In the era of computerized data bases and electronic dossiers, keeping a clean credit record means more than simply paying your bills on time. It means keeping tabs on those who keep tabs on you.

The reason is that while today's credit bureaus and their reports are a valuable tool for both lender and borrower, they are not perfect. Information is sometimes garbled; people with similar names can be confused with one another, and changes -- such as closing of a credit card account -- don't always get recorded promptly.

Thus, even the most conscientious consumers can find themselves with blemished reports. And they may not discover that there is a problem until they apply for credit -- a very awkward moment.

"One of the easiest mistakes to make is to assume that anyone with a problem with their credit report has somehow brought the problem on themselves," said Elgie Holstein of Bankcard Holders of America, a consumer group based in Herndon.

"Too often consumers assume that they don't have any problems" since they know they have paid all their bills, and are subject to "a last-minute surprise" when they are turned down for a loan, he said.

The consequences of these sorts of errors can be severe. As a congressional subcommittee heard earlier this month, getting a credit history straightened out can take months or even longer, sometimes causing borrowers to miss the chance to get a mortgage or other loan while rates are down.

The industry concedes that mistakes sometimes are made, but says that they represent only a tiny fraction of a vast volume of data that is provided to lenders. Walter R. Kurth of Associated Credit Bureaus Inc., a trade group, told the House consumer affairs subcommittee that much of the $720 billion in non-mortgage consumer credit outstanding today "was extended based on the information from a fast, accurate and pro-consumer credit reporting industry."

And he noted that erroneous reports serve no one's interest. "Obviously, the object of credit grantors is to make loans, not reject applicants," he said.

The actual level of error is subject to debate, but because so much data is collected and because it is so important to consumers, even if the number of mistakes is small, it behooves individuals to check the contents of their credit bureau files periodically.

"The single most important thing for people to do -- but most of us don't -- is to order a copy of your personal credit report," said Holstein. "It is critical that that be done on a regular basis, certainly prior to a major purchase involving credit. You need to know what the lenders know about you."

There are three major credit bureaus: Trans Union Corp., Equifax Inc. (which is also known as CBI or CBI/Equifax) and TRW Credit Data. In addition, there are many smaller firms; though most are either owned by or affiliated with the three major firms, others are specialty operations that provide highly customized data for clients such as mortgage lenders.

This means that if you turn to the Yellow Pages, you will find a large number of companies listed, but it's hard to tell which one you want.

The easiest approach, said Marvin Kaplan, a spokesman for Associated Credit Bureaus, is to "go to a bank or call the credit department of a major department store and tell them you would like to find out where your credit record is, that you are looking for companies that serve this community, and you would like to know if they can give you the names. ... I can't believe they would be so discourteous as not to give them to you."

Kaplan noted that you need not go physically to the bureau's office. You can call, find out what their fee is -- it ranges from $5 to around $15, depending on the state, but is free if you've been denied credit recently because of the bureau's report -- and send a check and a letter requesting your report.

And you will need to contact several offices because you want to make sure you see your Equifax, TRW and Trans Union files. They are likely to differ depending on which of their clients you do business with and what those clients choose to report.

Under the law, the bureaus are also required to explain their report to you. They are usually couched in symbols and codes, and while bureaus generally include a glossary, you may still find you need some assistance.

If you discover an error, you have the right to dispute it, and the bureau must recheck the item. If the item is reverified, you can have your dispute included in your file. But if you know the item is wrong, you should go to the source -- store, bank or whatever -- and ask them to correct it.

"Consumers need to be warned that they may need to be running back and forth between the subscriber {store or other lender} and the bureau," Holstein said. "You really have to cover both bases. Be prepared to shuttle back and forth." If the subscriber agrees to a correction, have them send you a copy.

If there is a legitimate black mark on your record, Holstein urged that you warn any potential lender before you apply for credit. If you can explain what happened and give reasonable assurance that it was an aberration, the lender may be willing to give it less weight in evaluating you.

Also, under the Equal Credit Opportunity Act, you have the right to submit an explanation and the lender must consider it. If you are turned down, you have the right to an explanation from the lender for a credit denial, and you should ask for it.

If you are turned down, or expect a problem, don't go all over town applying for credit, Holstein advised. Each application may trigger an inquiry and credit bureaus track these as well. Many lenders regard an excessive number of inquiries or credit applications as a bad sign.

To make all this easier, Holstein's group offers a "Credit Check-Up Kit" that includes a brochure on credit bureaus and consumers' rights and a list of credit bureau names, addresses and phone numbers for each region of the country. It's available for $2 by writing BHA/Credit Check-Up, 560 Herndon Pkwy., Suite 120, Herndon, Va. 22070.