It is well recognised that the newborn of diabetic mothers may be overweight and obese, presumably due to excessive glucose and insulin levels in the fetus. Since recent evidence indicates that the number of fat cells is established early in life, we studied the effect of intrauterine hyperglycaemia and hyperinsulinaemia on adipose tissue cellularity. Six men (age range 21–26 years) and six women (age range 18–24 years) were investigated. Their weights at birth generally exceeded the average value by 2 S.D. As a group they were not obese at the time of the investigation and neither total number of fat cells, average cell size nor body fat differed significantly from controls of the same age. There was no correlation between the number of fat cells and the weight at birth. The adipose tissue cellularity in the offspring of alloxan-diabetic rats (AX) and in controls (C) of equal weight was also studied. When sacrificed (after 40 days) body weights and the weights of the epididymal and retroperitoneal fat pads were similar in the AX and C groups. However, the number of fat cells of the retroperitoneal fat pad was significantly increased in the AX group, while the cell size was slightly diminished. Cell data of the epididymal fat pads were not significantly different. The results indicate that excessive glucose and insulin levels in utero may influence the number of fat cells, but, in man, they do not seem to lead to a permanent hyperplasia of the adipose tissue.