Sort of. The devices, which cost $100 and up, send a mild, unnoticeable electrical current up through one foot to the waist, then down the other leg to measure how much body fat a person has. Since fat is a poor conductor of electricity compared with water (the main component of muscles, liver, brain, etc.), the current flows more slowly through fat tissue. Using complex formulas that include age, sex, height, and activity level, the devices -- Tanita and HoMedics are the big players -- can thus calculate what percent of a person's weight is attributable to fat.

The devices are "surprisingly accurate," said exercise physiologist William J. Evans, director of the Nutrition, Metabolism, and Exercise Laboratory at the University of Arkansas for Medical Sciences. But the accuracy depends on hydration -- how much water you have in your system. "The things that can throw off the measure are excessive alcohol consumption and heavy exercise or sweating, in other words dehydration issues," he explained in an e-mail.

Dr. David Ludwig, director of the Optimal Weight for Life program at Children's Hospital is less sanguine about the scales. "The devices are moderately but not extremely accurate," he said. They "do give people a number, and lots of people like following numbers, so that may provide some motivation" for weight control. On the other hand, he said, many people are too obsessed with numbers on a scale already.

Personally, I would say save your money (or use it to buy some good walking or running shoes). Instead, calculate your BMI (body mass index), for which formulas abound on the Internet. Even better, take a tape measure and measure your waist and hips, then divide the first number by the second. For example, if your waist is 30 inches and your hips are 29, the ratio is 1.03. For women, if the result is over 0.85, you've got too much fat around the middle; for men, it's 0.95.