This paper reports on a longitudinal study of sleep problems in 200 children with severe mental handicap. Sleep problems were extremely common: 51% of children had settling problems, 67% of children had waking problems, and 32% of parents said they rarely got enough sleep. Sleep problems were also very persistent: between a half and two-thirds of children who exhibited sleep problems at Time 1 still had them 3 years later. Sleep problems were associated with a number of child characteristics: poor communication skills, poor academic skills, poor self-help skills, incontinence, daytime behaviour problems and epilepsy. There were no relationships with family variables such as social class, income, family composition or housing tenure. However, maternal stress, maternal irritability and perceived impact on the family were related to sleep problems. A Sleep Index was constructed, and path analysis was used to trace the main causal pathways of the child, family and social characteristics. Ten variables explained 50% of the variance in the Sleep Problems Index. Communication skills played a pivotal role. The implications of the findings for intervention strategies are discussed.