The prevalence of nocturnal enuresis (wet at least two nights a week) was investigated in children, aged 8, who were being followed up as part of a prospective cohort study. There were 175 children with homozygous sickle cell disease, 106 with sickle cell haemoglobin C disease, and 150 controls with a normal haemoglobin genotype. In homozygous sickle cell disease, 48 boys (52%) and 31 girls (38%) were enuretic, a significantly higher prevalence than in those with sickle cell haemoglobin C disease--five boys (10%) and 11 girls (20%)--or in normal children--16 boys (22%) and 13 girls (17%). There was no significant difference between children with sickle cell haemoglobin C disease and the normal genotype. Boys with homozygous sickle cell disease were significantly more likely to be enuretic if they came from large families; there was a similar trend for girls with homozygous sickle cell disease, although it did not reach significance. Enuresis was more common in boys with homozygous sickle cell disease who had low concentrations of fetal haemoglobin and in girls with sickle cell haemoglobin C disease who had high mean corpuscular haemoglobin concentrations. Similar associations were not shown for girls with homozygous sickle cell disease or boys with sickle cell haemoglobin C disease.