The development of nocturnal sleep of 79 mentally retarded children and adolescents (from 6 months to 20 years of age) was studied. Polygraphic recordings were carried out while the subject was in bed, and routine sleep parameters were measured. Total sleep time and percentage of Stage REM (rapid eye movements) decreased and awake time tended to increase with age. These values were similar to those previously found for age-matched nonretarded subjects. Results showed that the basic function of the sleep-waking system of retarded children seems to develop normally with age.