The measurements, taken over two years starting in 2001, are about equal to the 1999-2000 figures. In late 2001, then-Surgeon General David Satcher warned about the dangers of obesity.
The latest estimates were based on body measurements of 4,390 adults and 4,258 children. The survey, conducted regularly since 1960, is considered more reliable than studies in which participants report their own girth because people consistently underestimate their weight.
The findings appear in Wednesday's Journal of the American Medical Association.
"The level of obese and overweight Americans remains at alarming levels," said Allison Hedley, a researcher at the Centers for Disease Control and Prevention who led the study.
Before 1999, the results from the National Health and Nutrition Examination Survey were released in large chunks — such as 1988-94. But because obesity has become such a national concern, the data are being released every two years.
The survey found that obesity rates in several categories between 1999-2000 and 2001-2002 rose slightly, but in a statistically insignificant way. Because the time span is small, it would be wrong to conclude that obesity is leveling off, Hedley said.
Obesity is defined as having a body-mass index of 30 or above. The index is a measure of weight relative to height.
Patrick O'Neil, director of the Weight Management Center at the Medical University of South Carolina, said he was most bothered by the numbers of overweight children.
The study found that 31.5 percent of children ages 6 through 19 were overweight in 2001-02, and 16.5 percent were seriously overweight, or obese.
"We're producing a new generation that's even more vulnerable to obesity and all of its health consequences by virtue of the fact they're starting out more overweight than previous generations," O'Neil said.