It wasn't long ago that the quest for self-knowledge meant analyzing the psyche as opposed to, say, the colon. But that was before Katie Couric showed us hers on national TV. Now, for those who can afford it, knowing thyself often means knowing, quite literally, the stuff of which you are made. It means availing yourself of the newest and most intrusive technology to find out virtually everything there is to know about your physical being.

Consider the flourishing ritual known as the executive physical, a perk that about half of all large American companies offer their top managers each year, though many doctors say there is no medical benefit to such frequent and comprehensive exams. Recipients of this largess are dispatched to "executive health programs" at deluxe venues like the David Drew Clinic in Chevy Chase, Md., where they spend a day submitting to "the latest high-tech medical approaches in the surveillance of occult illness, including genetic testing, body imaging and state-of-the-art disease markers."

Such thorough plumbings of your inner space cost around $2,700. Fortunately, the rest of us can undergo the downmarket versions of aggressive screening. Preventive CAT scans -- total body imaging of healthy people, at only $600 or so -- are becoming increasingly popular, even though most doctors consider them ill advised. Meanwhile, new companies like HealthScreen America offer consumers the chance to order up their own diagnostic exams -- for everything from prostate cancer to stroke risk -- get the tests done at their local mall, then download the results from the Internet. Such direct-to-consumer exams are sure to multiply in the future, and it won't matter if their medical value remains contested. They're a hypochondriac's dream -- the means to investigate every little bleep of a symptom, without ever being brought to your senses by an actual doctor.

Would-be Howard Hugheses, your Shangri-La awaits. Imagine home CAT scans in the privacy of your own marble bathroom. Automatic urine analysis with every flush. Computerized life-expectancy projections updated after each meal. Home exercise equipment that transmits your blood pressure and heart-rate readings directly to a lab (and maybe to your insurance company too) every time you break a sweat. Bedside spirometers for measuring lung capacity before, during and after sex.

Who will want this stuff? There will be people who see a value in knowing exactly what their lipid profile says about them, and people who, more fatalistically, do not. And this division will likely map onto pre-existing conditions of class. Those who want to know will tend to be the wealthy and educated, people who assume that money and information well deployed can save them from almost anything. On the other hand, in a recent study involving family members at risk for a hereditary colon cancer, 60 percent did not want to know the results of a test that would reveal whether they had the relevant DNA mutation. The less formal education they had, the less likely they were to want to know.

Moreover, these new screening opportunities for the worried well will cost the rest of us something, depleting the resources available for more practical preventive health measures -- smoking cessation or weight-loss programs, for example. "Somebody who's 25 and thinks: Hey, what's the problem? I'm paying for my own mammogram, sets into motion a whole chain of things," says Dennis Fryback, a professor of preventive medicine at the University of Wisconsin. "There will be additional tests, biopsies. Insurance pays for those, which raises everybody else's premium. And we always get many more false positives than true ones."

There are screenings, of course, whose preventive health benefits are known and proven -- TB tests, mammograms for women over 50, pap smears. But there are others -- mammograms for men -- that are dubious because while they could benefit somebody (a few men do get breast cancer) they would do so only at tremendous cost in both dollars and anxiety to a large population of extremely unlikely victims. And there are still others -- full-body CAT scans, large-scale testing for prostate cancer -- that are dubious because what they turn up may be a very slow-growing tumor or an "indolent" disease that would never have caused a problem if the patient hadn't known about it. Only now that he does know, he may have unnecessary surgery (which carries its own risks). Or he may move through life burdened by the sense that he is a future patient, an illness waiting to happen.

The truth is, it is not always easy to put such portentous knowledge to rational use. Consider this cautionary tale. Some years ago, Sweden established an experimental program in which about 200,000 newborns were screened for a recessive genetic condition that would predispose them to emphysema as adults. Since exposure to cigarette smoke, dusty environments and the like would increase the chances of contracting the disease, doctors anticipated that families whose children had this genetic predisposition could change their lifestyles accordingly. Instead, the whole program had to be canceled because families experienced such high levels of emotional disturbance. Parents reported that they couldn't help thinking of their affected children as "different"; many of them said that their anxiety about the disease lying in wait led them to smoke more, not less. In the end, maybe there is such a thing as too much information -- a diagnostic dystopia in which an anxious few of us are privileged to know too much about what we are made of, and too little about what it means.