A 9-year-old, blind boy with severe mental retardation with a chronic sleep/wake disturbance had a circadian rhythm of 24.75 hours and an internal desynchronization of the endogenous rhythms. Treatment with oral melatonin given at 6 PM induced a regular sleep/wake pattern. Melatonin, in this patient, convincingly entrained the endogenous rhythm to the appropriate chronological 24-hour day.