Six of 10 blind subjects had unusual melatonin circadian secretory patterns. One of these subjects, studied longitudinally, appeared to have a free-running melatonin secretory rhythm with a period of 24.7 h. Another subject, also studied longitudinally, appeared to secrete melatonin during the day (instead of during the night). Therefore, some blind subjects appear to have different types of circadian rhythm abnormalities. These findings along with previous work demonstrating suppression of human melatonin secretion by light suggest that the light-dark cycle may be important in the regulation of the human melatonin circadian secretory rhythm. These findings may stimulate more research on the endocrinological consequences of blindness as well as on the biological effects of light in humans.