Adolescence in girls is a critical period for the development of obesity. Reduced energy expenditure represents a strong risk factor. We are continuing to follow annually a cohort (originally 197 pre- menarcheal girls) who were enrolled (1990-1993) in a longitudinal study of energy expenditure and its relationship to the development of obesity. One hundred and fifty-nine girls (80.7%) remain active in the study. Of these 159 subjects, 59 have completed the study and the other 100 continue to seen annually. A baseline measures of body composition, resting metabolic rate, total daily energy expenditure, and VO2 max were completed. Measures of diet and activity were obtained from a 7-day food record, a 7-day activity record, and activity questionnaires. Since enrollment, girls are seen annually until 4 years after menarche when they exit the study. At their annual visit, measures of diet, activity, growth and body composition by anthropometric measures are obtained. Blood is drawn for the measurement of sex hormones and insulin. At the end of this proposed grant period, all but five girls will be 4 years post menarcheal and will exit the study. At that time we will have determined whether reductions in energy expenditure at baseline (total daily energy expenditures, resting energy expenditure, percent non- resting energy expenditure or VO2 max) are associated with increases in body fat as girls mature. We will also determine whether differences in ethnicity, or parental obesity contribute to the changes in body fat in a subcohort of girls, visceral fat was previously measured at menarche by MRI. At the time of the MRI, measures of body composition and anthropometry were repeated, blood was drawn for the measurement of sex hormones, and questionnaires were completed rating to diet, activity and behaviors such as smoking and alcohol abuse. We will repeat these measures in these girls at their exit visit. This study will allow us to determine what factors contribute to visceral fat deposition in girls during puberty. These studies will clarify the role of energy expenditure in the development of obesity and identify factors that contribute to the deposition of visceral fat. Identification of factors which contribute to the development of obesity and central fat distribution will aid in the development of programs designed to reduce the incidence and morbidity associated with adolescent obesity.