Lately it has been in vogue to dismiss the advice to drink eight glasses of water a day as a 'medical myth.'

Books and medical-journal articles have declared there's no scientific evidence for claims that '8 x 8' -- eight ounces, or 237 milliliters, of water eight times a day -- can bring a wide range of benefits, from speeding weight loss to ridding the body of toxins, fighting constipation, fatigue and dry skin and hastening recovery from colds and the flu. Headlines have jeered that 8 by 8 'doesn't hold water' and 'water advice doesn't wash.'

It's really more a dispute over whether the glass is half-empty or half-full.

Many studies have linked drinking extra water with health benefits, but critics generally dismiss them as statistically insignificant, inconclusive or not widely applicable.

For instance, a 10-year study of nearly 48,000 men published in the New England Journal of Medicine in 1999 found that the risk of bladder cancer fell 7% for every cup subjects drank per day. Other studies have found that the more water subjects drank, the fewer precancerous colon polyps they had. And a study of 20,000 Seventh-day Adventists in California in the American Journal of Epidemiology found that women who drank at least five glasses of water a day had a 41% lower risk of fatal heart disease, and men had a 54% lower risk, compared with those who drank just two glasses a day.

Physiologist Heniz Valtin of Dartmouth Medical School discussed these studies and more in a seminal 2002 article in the American Journal of Physiology -- and still concluded that they didn't support a universal recommendation that everyone drink 8 by 8, just people 'known to have a propensity for the disease(s) in question.' Six years later, Dr. Valtin says, 'I haven't seen a single scientific report that disagrees with my conclusion.'

Similarly, an editorial in the Journal of the American Society of Nephrology in April brushed off small studies suggesting that water increases thermogenesis (calorie burning), reduces migraines and increases blood flow to the skin. The piece repeated Dr. Valtin's conclusion: 'There's no clear evidence of benefit from drinking increased amounts of water.' But, it added, 'we concede there is also no clear evidence of lack of benefits.'

That frustrates researchers in the field. 'There's enough data to suggest that we should go get more data,' says Jodi Stookey, a scientist at Children's Hospital and Research Center in Oakland, Calif. Her studies have found that substituting water for sweetened beverages helped female dieters cut overall calories (whereas diet drinks seem to stimulate more eating) and that women on four popular diets who drank at least one liter of water a day lost more weight than those who didn't, regardless of their diet.

It's impossible to sort out cause and effect from casual associations in such studies, and participants aren't always accurate when asked to remember what they ate or drank. So what would satisfy skeptics as evidence? 'Only large and expensive randomized trials could settle these questions definitely,' the April editorial notes. 'Given that water cannot be patented, such trials seem unlikely.'

Urologists do agree that extra water can reduce the recurrence of kidney stones. But they don't know if water will prevent them in the first place. 'Kidney stones occur in 4% to 5% of the population, so it's not practical to tell everyone to drink that much,' says Stanley Goldfarb, a kidney specialist at the University of Pennsylvania Health System and co-author of the editorial.

He also disputes the notion that drinking extra water improves the function of various organs -- because it doesn't stick around. 'You don't accumulate those eight glasses of water. They're in the toilet,' he says, noting that the body has an exquisite system of self-regulation, excreting anything it doesn't need. And that doesn't mean the body is flushing out extra toxins, either, he says; the same toxins are just diluted in more water.

Experts also agree that people's water needs vary considerably: you'll need more if you're playing in the U.S. Open than if you're sitting at a computer in an air-conditioned office. But most nephrologists, and the National Academy of Sciences, say that thirst alone is a perfectly sufficient guide to how much you need. 'Thirst is one of the most powerful human motivators -- it's way above sex,' says Dr. Goldfarb.

Actually, that's a point of contention, too. Fereydoon Batmanghelidj,an Iranian-born doctor who extolled the benefits of water in two books, called relying on thirst to regulate hydration 'the greatest tragedy in medical history.' He and other water aficionados believe that by the time people feel thirsty, they are already dehydrated. This camp believes that the body needs roughly two liters of water a day to replace what it loses and that other beverages, particularly caffeinated ones, don't count.

A variety of official government recommendations concur with the two-liter total but say that includes the considerable amount of water that is naturally found in solid food. 'Even a slice of white bread is more than 30% water,' says Dr. Valtin.

Still, some diet experts firmly believe that drinking extra water helps people feel fuller and makes the body retain less fluid, even though some concede the benefit may be as much behavioral as metabolic.

'If your usual habit is to have a bowl of popcorn or peanuts on your desk, sipping on a glass of water instead is satisfying and keeps your hands and mouth busy,' says Karen Miller-Kovach, chief scientific officer at Weight Watchers, which recommends six glasses a day.

Some marathoners have died from downing large quantities of water very quickly. The human body can absorb only about one quart of water per hour, max, and after that, the brain can swell dangerously, says Dr. Goldfarb. Another danger is hyponatremia, in which sodium levels fall precipitously -- but that's also rare in healthy people. Drinking eight glasses a day 'is not likely to harm you, but it's very unlikely to help you in any way that the conventional wisdom has been claiming,' says Dr. Goldfarb.

Some boosters, however, say they have all the evidence they need. 'When I'm drinking a lot of water during the day, I feel better,' says Theo Robbins, a Manhattan attorney. 'I'm less inclined to get a headache. I have more energy. I'm able to focus a little better, and those are important things.