Measurements of total body potassium in obese children were made in the whole body scintillation counter, and from this value lean body mass was calculated, and then fat by difference. A series of normal children was available for comparison. There was a good correlation between total body fat and relative body weight, abdominal circumference, and buttocks circumference, and a fair correlation with triceps skin-fold thickness. Although the series is a small one, it suggests that there are two types of obesity in childhood. The first is characterized by increased LBM in addition to fat, by a tendency to tallness and advanced bone age, and to have been overweight since infancy. The second group shows no increase in LBM; these subjects tend to have a normal bone age, to be of average height, and the majority have become obese during the childhood years.