Fourteen children aged 9 months-4 years with moderate to severe mental retardation and varying neurologic lesions were referred for severe and continuous nocturnal sleep disturbances and very abnormal day/night schedules. All children had previously been given hypnotic medications and behavioral treatments which had little or no effect on nocturnal sleep. The severity of the sleep disturbances significantly affected family life and was a major handicap to the children. All children were treated with light therapy (minimum 4000 lux). Five children responded to treatment and had normal sleep-wake cycles at the most recent post-treatment evaluation (2-5 years after the first treatment). Two of the patients' families were unable to follow the prescribed regimen. Treatment failed in 7 children. One of these seven children spontaneously improved 3 years later. In three of the failure children the neurologic problem progressively worsened, leading to death in one of them. Phototherapy is a treatment worth pursuing in children with very significant sleep/wake disruption which is unresponsive to behavioral or other treatments. It has few side-effects and may lead to normalization of the sleep-wake cycle. Recent improvement in the technology used to monitor the 24-h temperature rhythm over several days and the present commercial availability of "light boxes" should render these therapeutic trials easier than at the time of these initial investigations.