Body fat and adipose tissue fat cell size and number were determined in a group of obese patients and in controls. These variables were analyzed at different degrees of obesity and compared with metabolic variables, including blood glucose and plasma insulin during glucose tolerance testing and blood lipids. Fat cell size was responsible for the increase of obese adipose tissue at a moderate degree of obesity. With more severe obesity, fat cell number becomes increasingly important and dominates as a factor contributing to obesity. Under conditions of unrestricted diet and activity, plasma insulin correlated positively with fat cell size but not with body fat, and tended to show a negative correlation with fat cell number. The factor in adipose tissue associated with plasma insulin increase is thus probably the fat cell size. The results suggest two forms of obesity. One is characterized by a hypertrophy of fat cells and is of a moderate degree (hypertrophic obesity). This type of obesity is associated with metabolic disturbances. Increased fat cell size might not be a primary factor in this form of obesity, but rather another symptom of metabolic disturbance. The other form (hyperplastic obesity) has an increased number of fat cells and is associated with much more severe obesity, particularly since fat cell size is often increased also in these patients.