Changes in body fat mass were studied in 25 untreated patients with Laron syndrome from childhood into adulthood. It was found that these patients, characterized by marked dwarfism, high plasma hGH and low serum insulin-like growth factor I (IGF-I), develop progressive and marked obesity and have a tendency for elevated serum cholesterol levels. Long-term treatment of 8 children and 5 adults with this syndrome with IGF-I (50-150 micrograms/kg/day s.c.) resulted in a significant decrease in subcutaneous fat in all patients and a lowering of the serum cholesterol and triglycerides, mainly in the adults. As in Laron syndrome the GH receptors are inactive, it is hypothesized that IGF-I exerts a direct effect on adipose tissue metabolism.