A random sample of 3,206 seven-year-old children was studied in order to examine the prevalence of enuresis and associated somatic and genetic risk factors. The overall prevalence of enuresis was 9.8% and the figures for nightwetting, daywetting and mixed day and night wetting 6.4%, 1.8% and 1.6% respectively. The prevalence was 9.5% among primary school children, 24.8% among children whose entry to school had been postponed and 26.6% among handicapped and mentally retarded children. If the father had been enuretic after 4 years of age the risk of the child being enuretic was 7.1 times greater than otherwise (95% confidence limits of the risk ratio 5.1-9.8, p less than 0.001), the corresponding risk ratio when the mother had been enuretic being 5.2 (3.9-7.0, p less than 0.001). Low birth-weight children were enuretic more often than children of normal birth-weight. It seems that there are at least two aetiologically significant groups of enuretic children: cases with neurological damage and mixed day and night wetting and cases with delayed maturation, with nightwetting which shows a clear sex and genetic dependency.