Oral melatonin therapy was used to treat-severe circadian sleep-wake disturbances in eight children and young adults in an open study. All patients were functionally blind, six of them because of defects in the anterior visual pathway. All were mentally retarded. Baseline sleep diaries kept by the caregivers before treatment showed non-24-hour sleep-wake syndrome. Diurnal variations in serum and urinary melatonin were examined. Melatonin secretion peak time was delayed in seven patients. Body temperature variation was out of phase relative to sleep and melatonin in five patients, and thus they had signs of internal desynchronisation. Melatonin given in the evening dramatically improved the sleep-wake pattern in all patients. The effect was maintained during long-term therapy for between 1 and 6 years in six patients. One patient fell back into the earlier sleep pattern after 6 to 8 months, and another had increasing sleep disturbance because of reflux oesophagitis, although the improvement regarding the circadian component remained. No side effects have been noted during the therapy. Oral melatonin is promising as an efficient and seemingly safe alternative for treatment of severe circadian sleep disturbances.