To determine the effect of hypercortisolaemia on the melatonin circadian secretion 12 patients with pituitary or adrenal dependent Cushing's syndrome and 5 healthy controls were studied. The melatonin circadian rhythm of secretion, observed in the control group, was abolished in the patients with hypercortisolaemia. Mean nocturnal melatonin levels and the integrated 24-hour secretion were significantly lower in the patients studied than those of the controls. Thus, in patients with Cushing's syndrome the melatonin levels are decreased and the circadian rhythm of this hormone is abolished.