The data come from thousands of children who have taken part in the National Health and Nutrition Examination Surveys  compiled by the National Center for Health Statistics at the C.D.C. since the 1960s  and represent some of the most reliable statistics available on the health of American children.

The most recent data is based on two surveys  one in 2003 to 2004 and one in 2005 to 2006  that included 8,165 children ages 2 to 19. In that group, about 16 percent of children and teenagers were obese, which is defined as having a body mass index at or above the 95th percentile on United States growth charts. For example, a 10-year-old girl who is 4-foot-7 would be considered obese if her weight reached 100 pounds. By comparison, about 5 percent of children and teenagers in the United States were obese in the 1960s and 1970s. As startling as those numbers are, the good news is that from a statistical standpoint, obesity rates have not increased since 1999. Estimates for the number of children who fall into the overweight or obese category also have remained stable at about 32 percent since 1999. Overweight is defined as at or above the 85th percentile.

In fact, the number of children who fall into the obese category decreased from 17.1 percent to 15.5 percent between the 2003 and 2006 surveys, but the decline was not statistically significant. So the researchers combined data from both surveys to enhance the statistical strength of the numbers.

The plateau follows years of excessive weight gain among American schoolchildren. For instance, in 1980, 6.5 percent of children age 6 to 11 were obese, but by 1994 that number had climbed to 11.3 percent. By 2002, the number had jumped to 16.3 percent, but it has now appeared to stabilize around 17 percent.

“It doesn’t mean we’ve solved it, but maybe there is some opportunity for some optimism here,” said Cynthia Ogden, the lead author of the journal report and an epidemiologist for the National Center for Health Statistics.