If you've had occasion to spend a few days in the hospital recently, you've probably also had occasion to gaze at one of the most incomprehensible documents of modern times -- a hospital bill.

Only a few days of treatment is often enough to result in page after computer-generated page of jargon and code. And even if you know what it means, you may not know whether you really received all the items you are billed for. Under anesthesia during surgery or unconscious in the emergency room, you are hardly in a position to inventory the services being rendered.

"Almost everyone, at one time or another, has received a medical bill and paid it without the slightest idea nor the capability of understanding what they are paying for," said Sen. William V. Roth (R-Del.), ranking minority member of the Senate permanent investigations subcommittee.

It turns out that anywhere from 2 percent to 99 percent -- depending on who's counting and at what hospital -- of hospital bills contain errors. Sometimes the errors favor the patient; sometimes they favor the hospital.

"Hospitals historically have been, shall we say, sloppy when it comes to their internal accounting systems," said Richard Landen, associate director of the Health Insurance Association of America, a medical insurance trade group here.

The situation has become bad enough to cause medical insurance companies to set up elaborate computer systems that use "protocols" to screen out charges that may not be justified. At the same time, it has spawned an industry of firms that contract with hospitals to re-audit patient records and look for additional money to be collected.

Both sides, naturally enough, say they are only trying to make sure the other doesn't get anything it shouldn't.

The re-audit companies, known as "revenue recovery" firms, look over old bills for items that were omitted or under-billed and crank out a new bill to send to the patient and the insurer. These firms are supposed to check just as hard for undercharges, but since they often operate on contingency -- taking a percentage of additional payments they generate and giving the rest to the hospital -- questions have been raised about whether they really do that.

The money at issue is not small potatoes. According to the Congressional Research Service, private insurers pay $75 billion in hospital claims annually. And as employers tighten up on their benefits packages in an effort to control rapidly rising costs, more and more individuals find themselves obliged to pay at least a portion of the charges billed to them for medical costs.

The relationship between hospitals and insurers today amounts to "outright war," according to officials of Manns Co., a California-based revenue recovery firm.

With insurers and hospitals at each others' throats over every dollar, more and more people are finding themselves, as Robert Hunter of the National Insurance Consumer Organization put it, "trapped in the protocols" -- with the hospital demanding payment and the insurer refusing to comply.

"It's a serious problem," Hunter said.

He noted, however, that if an insurer balks at paying a charge that seems legitimate, patients have well-established procedures available to them to contest an insurer's refusal to pay a claim. A refusal to pay can be appealed within the company, then to the state insurance department and ultimately to the courts, Hunter said, and where there is any ambiguity, courts tend to look at what might reasonably be expected by the insured.

On the other hand, if the hospital appears to be overcharging, patients can expect their insurance company to resist unexplained or unreasonable charges as well because in most cases the insurer will have to pay the lion's share of them.

But evidence unearthed recently by the permanent investigations subcommittee suggests that bills received after the account had presumably been closed should be viewed with more than usual vigilance. That is because these revised bills are often the work of a revenue recovery firm.

Witnesses who have worked for or with such companies told the panel that they are commonly given quotas to meet, told to report only items where the patient was undercharged and not where he or she was overcharged, and in some cases told to assume a treatment that would normally be administered was in fact administered even if no record of it appeared on the patient's charts.

Congress's General Accounting Office went back over a sample of bills audited by one revenue recovery firm. While the firm had found that 15 percent of the bills contained overcharges -- meaning that the patient (or insurer) was due a refund -- GAO found that 99 percent of the bills contained such overcharges, according to a report by the subcommittee's minority staff.

The revenue recovery companies counter that they are merely responding to unreasonably restrictive payment policies by insurers that are strangling hospitals.

Insurers have "instigated a campaign of business terrorism and sabotage to put us out of business," said Manns officials in a statement to the subcommittee.

But if revenue recovery firms are inflating hospital bills, the costs will be felt by the patients even if their insurers pay all of the extra charges. Such charges will inevitably find their way into the insurer's cost calculations and from there to higher premiums.

"In the end, one way or another, the patient ends up paying," Roth said.