Patients with craniopharyngioma frequently suffer from severe obesity. Leptin induces an inhibition of appetite via hypothalamic receptors. This study was undertaken to investigate whether a relationship exists between serum leptin levels and pituitary/hypothalamic lesions in craniopharyngioma patients. Serum leptin levels were evaluated by RIA in 14 patients (age 7-21 years; 7 females, 7 males) after they had undergone neurosurgical treatment for craniopharyngioma. Normal controls had a positive correlation between leptin levels and body mass index (BMI) with higher levels in the females than in the males. Significantly elevated leptin levels with respect to BMI were found in 11 craniopharyngioma patients who had been affected by a suprasellar tumour, whereas 3 patients with an intrasellar tumour had lower, almost normal serum leptin levels. Our data suggest that craniopharyngioma patients develop hypothalamic obesity because their hypothalamic structures are insensitive to endogenous leptin. The elevated serum leptin concentrations found only in patients with a suprasellar tumour may be explained by a disturbed feedback mechanism from the hypothalamic leptin receptors to the adipose tissue.