The effects of weight reduction on reproductive hormones were investigated in 24 moderately obese men, 18-108% above ideal body weight. Serum estrone (E1), estradiol (E2), testosterone (T), percent free T (%FT), sex hormone binding globulin (SHBG) capacity, and, in 9 subjects, androstenedione (A) were measured serially before and during an outpatient supplemented fasting program (320 kcal/day) for 8-20 weeks. In the baseline state mean E1 was elevated to 100 +/- 7 pg/ml (normal, 30-60 pg/ml). The E2 was slightly elevated to 36 +/- 3 pg/ml (normal, 8-35 pg/ml). The mean T of 400 +/- 20 ng/dl was at the lower end of normal (400-1000 ng/dl). The mean %FT was elevated to 4.1 +/- 0.2% (normal 1.6-3%). The calculated free T was normal. The mean SHBG binding capacity was 0.99 +/- 0.05 micrograms dihydrotestosterone bound/dl (normal, 1.0-1.8 micrograms/dl). The mean A of 52 +/- 5.8 ng/dl was normal. These data were in accord with previous findings in much heavier men. Eight weeks of weight loss (mean, 19.5 kg) were associated with normalization of all the measured parameters. The mean E1 decreased to 48 +/- 23 pg/ml, E2 to 28 +/- 2.1 pg/ml. T increased to 536 +/- 35 pg/dl and %FT fell to 3.2 +/- 0.2%. Data on men remaining on the program for 16 or 20 weeks showed a continued fall of estrogens and stabilization of T and %FT. SHBG and A did not change significantly over the entire time period. In conclusion, increased circulating estrogens and reduced androgen binding were found in moderately obese men, which were completely corrected with weight loss.