The proportion of American children who are overweight has increased more than 50 percent over two decades, and the nation is facing an epidemic of childhood obesity, according to a study directed by a School of Public Health professor.

The study--directed by Dr. Steven L. Gortmaker, associate professor of sociology--documents a striking increase in weight problems among both grade school children and teenagers.

Obesity is particularly common among white youngsters, although the number of obese Blacks is increasing. The study estimates that nearly a third of all white boys in their pre-teen years are overweight.

The researchers found that between 1963 and 1980, there was a 54 percent increase in the prevalence of obesity among children between ages 6 and 11, and there was a 39 percent rise in obesity among adolescents between 12 and 17.

"Childhood obesity is epidemic in the United States," said Dr. William H. Dietz Jr. of New England Medical Center, a co-author of the study, which is being published in the May issue of the American Journal of Diseases of Children. "The implications are that there is going to be a major rise in the prevalence of adult obesity and its consequences."

The study found that the likelihood of obesity also varies among social classes and regions of the country. "The fattest kids in the United States are located in large urban areas in the Northeast," Gortmaker said. The skinniest ones live in the West.

Poor children are thinner than middle-class youngsters, although the poor are quickly narrowing the weight gap. Children and teen-agers also tend to be fatter in the winter than in the warmer months.

The study was based on an analysis of data on 21,680 children that were compiled by the National Center for Health Statistics. The experts estimated youngsters' obesity by measuring the amount of fat in their arms.

They found that by 1980, 27 percent of those 6 to 11 and 22 percent of those 12 to 17 fit this definition of obesity.

The fattest 5 percent of the youngsters in 1960 were considered by the researchers to be "superobese." In 1980, 9 percent of the teen-agers and 12 percent of the preteens were in this category.

The researchers did not use total body weight to determine obesity, because it's hard to compare youngsters who are different heights. However, in general, experts consider people to be obese if they are more than 20 percent above their ideal body weight. Dietz said he could not estimate how far over ideal body weight the super-obese youngsters were.

Dietz said that few health problems are associated with young people's obesity, but that it can be a sign of trouble to come.