Urinary melatonin levels were measured in 10 postmenopausal Indian women suffering from advanced stages of breast cancer and in 9 well-matched women with non-endocrine complaints, mostly uterovaginal prolapse. Urines of each patient were collected over a period of 2-3 days in four 4-hourly intervals from 6 a.m. to 10 p.m. and one 8-hourly interval from 10 p.m. to 6 a.m. Serum LH, FSH, prolactin, estradiol and cortisol levels at 11 a.m. were determined as well as estrogen and progesterone receptors of the breast tumors. It was found that 24 hour urinary melatonin excretion in cancer patients was on the average 31% decreased as compared to the controls. This change was accompanied by a 33% increase in serum cortisol levels in the cancer patients. The melatonin excretion patterns of the cancer patients were not synchronized as compared to synchronized patterns of the controls. The number of tumors tested for steroid receptors does not yet allow to conclude if melatonin is different in patients with or without hormone-dependent tumors. The data suggest that pineal melatonin secretion may be modified in quantity as well as rhythmicity in breast cancer patients.