Cushing's syndrome (CS) is associated with weight gain and visceral obesity. We examined the relationship between regional fat distribution and serum levels of leptin, cortisol and insulin. Twenty-three consecutive patients with recently diagnosed CS (18 with pituitary adenoma, 5 with adrenal tumor), where compared to obese controls, matched for age, sex and Body Mass Index (BMI). Serum insulin, leptin, cortisol, C-peptide and body composition determined by DEXA were measured. Serum leptin levels were significantly increased in patients with CS (36.9+/-3.8 vs 18.9+/-2.4 ng/ml, p<0.001; women: 40.1+/-4.6 vs 21.7+/-2.9 ng/ml, p<0.01; men: 27.9+/-5.7 vs 10.9+/-2.3 ng/ml; p<0.05), the same were fasting insulin levels (178+/-30 vs 81+/-10 pmol/l; p<0.01) and C-peptide (1.51+/-0.12 vs 0.77+/-0.07 nmol/l; p<0.001). In a subgroup of 12 patients, truncal fat mass was significantly elevated when compared to obese controls (19.2 kg vs 14.7 kg, p<0.01, and 42% vs 36% in percentage of truncal body tissue, p<0.05), whereas total fat mass was insignificantly increased. Serum leptin correlated positively to total body fat (%) as in patients with CS (r=0.94, p<0.001) as in controls (r=0.68, p<0.01). The correlation to truncal body fat (%) was also significant in both groups (CS: r=0.84, p<0.001; controls: r=0.63, p<0.01). Multiple regression showed that percent total body fat was the predictor of leptin concentrations among patients with CS (r2=0.88, p<0.001) whereas insulin did not contribute significantly to the variance in leptin concentrations. In controls, both leptin and insulin (r2=0.65, p<0.001) contributed significantly to the variations in leptin levels. Controlled for the differences in total body fat, patients with endogenous CS have significantly increased serum leptin levels, compared to BMI-matched obese controls. This suggests that hyperleptinemia in CS not primarily reflects changes in body composition, but is the result of different hormonal influences on adipose tissue.