The possibility that chronic nutritional deficiency alters leptin regulation and its link to reproductive function was investigated by determining serum leptin levels during a 24-h period with controlled nutrient intake in highly trained athletes with and without menstrual cyclicity and in BMI-matched cycling sedentary controls (n = 8 per group). Our data show that 24th leptin levels were reduced equally (3-fold, P < 0.001) in both cyclic and amenorrheic athletes as compared to controls. Low leptin levels in the athletic groups were consistent with their reduction in body fat (r = 0.91, P < 0.0001) relative to BMI, but were also influenced by the presence of low insulin (r = 0.70, P < 0.001) and elevated cortisol (r = -0.65, P < 0.001) levels. A diurnal pattern of 24h leptin levels, with an approximate 50% rise (P < 0.001) from nadir (0900h) to peak (0100h), was present in normally cycling athletes and controls and was strikingly absent in amenorrheic athletes. The absolute increase in leptin levels from nadir to peak was directly related to insulin excursions in response to meals (r = 0.60, P = .002) and inversely related to the amplitude of the 24h cortisol rhythm (r = -0.70, P = .0002). These findings are consistent with a link between the functionality of adipocytes, nutritional status, and integrity of the reproductive axis in humans.