We tested hypothalamic, pituitary and endocrine function in 19 patients with secondary amenorrhea associated with simple weight loss who did not have anorexia nervosa to evaluate the effects of weight loss on these systems. Thermoregulation at 10 degrees C and 49 degrees C was abnormal and correlated with the percentage below ideal body weight (r = 0.62, P less than 0.02, and r = 0.55, P less than 0.05, respectively). Partial diabetes insipidus was found in 27 per cent of patients with simple weight loss. They had delayed peak plasma luteinizing hormone levels after 10 microgram of luteinizing-hormone-releasing factor, which was correlated with percentage below ideal body weight (r = 0.49, P less than 0.05). Delayed peak plasma thyrotropin levels after 500 microgram of thyrotropin-releasing factor were found. No prolactin, pituitary, thyroid or adrenal abnormalities were present. These findings are qualitatively similar to results of studies in 29 patients with anorexia nervosa, but less severe and less frequently present. We conclude that hypothalamic dysfunction may be caused by weight loss per se.