Circulating leptin shows a pulsatile secretory pattern along with a nocturnal rise. We have previously shown that circulating leptin concentrations are high in males with untreated idiopathic hypogonadotropic hypogonadism (IHH). However, circadian leptin secretion in IHH before and after gonadotropin treatment is not known. Thus, we studied 14 adult males with IHH who had no history of previous hormonal therapy, and 12 age- and body mass index-matched healthy men. Plasma leptin concentrations were measured with 1-h intervals for 24 h before and 6 months after gonadotropin treatment. The 24-h mean leptin concentration showed a significant decrease, from 11.78 +/- 1.908 microg/liter at baseline to 10.85 +/- 1.939 microg/liter after 6 months of therapy (z = 3.107; P = 0.002). Before and after treatment, 24-h mean leptin concentrations were also significantly higher in the patient group when compared with controls (4.275 +/- 0.711 microg/liter) (z = 5.938; P = 0.0001). Hourly leptin levels demonstrated a diurnal pattern in hypogonadal patients, a surge in the midday, and a peak just after midnight, and this pattern did not differ before and after treatment. We observed a similar diurnal pattern in the control subjects too. Leptin levels were negatively and significantly correlated with free testosterone and total testosterone levels both before (r = -0.656, P = 0.011; and r = -0.639, P = 0.014, respectively) and after (r = -0.537, P = 0.048; and r = -0.563, P = 0.036, respectively) gonadotropin administration. Our observations suggest that the diurnal rhythm of leptin is intact in males with IHH, and short-term gonadotropin treatment does not effect its diurnal rhythm. Moreover, testosterone produced under the influence of the gonadotropin treatment led to decreases in the leptin levels.