The aim of this population-based multicentre study was to evaluate the prevalence rates of sleep problems among 8-9-y-old children. The sample consisted of 5813 Finnish children, making up 10% of the age cohort. Both parents and children provided information. Disturbed sleep was reported by 21.7% of parents. Most of the problems were mild; only 0.3% were serious. Dyssomnias were frequent: 11.1% had difficulties with sleep onset, 7.1% with night waking and 2.3% with waking too early. Multiple sleep problems were present in 9.1% of the children. 17.8% of children reported disturbed sleep, 12.7% had problems many nights and 5.1% every night. In 32.0% of cases, either the parent or the child reported disturbed sleep; 7.4% of these reports came from both the parent and the child, 14.1% from the parent only and 10.3% from the child only. The correspondence between informants was poor (kappa = 0.224). Sleeping problems were associated with somatic and psychiatric problems. It is concluded that by restricting questioning to parents only, one-third of all potential cases of sleep problems may go unnoticed. In order to increase the sensitivity of screening children's sleep problems, both parents and children should provide information in epidemiological settings as well as in clinical work.