The epidemiology of snoring and sleep disordered breathing was studied by surveying 996 4 to 5 year old children living in Oxford. Responses were obtained from 782. Habitual snoring was common at 12.1% and was associated with sleep and behavioural problems. Two years later the same group were restudied. Snoring and associated problems had resolved in approximately half the children, but the overall prevalence of snoring was virtually unchanged because an equal number had developed the symptom anew. Children who snored habitually in both surveys had significantly more sleep and behaviour problems than those who had developed snoring more recently, suggesting that chronic snoring may adversely affect children's behaviour and development. Two subgroups (66 children in each) from the original population of children were studied at home with overnight oximetry, video recording and behavioural assessment using the Conners Behaviour rating scale. From this study the lowest prevalence of significant sleep disordered breathing was 1.1% in this population. Children with symptoms suggestive of sleep disordered breathing had more sleep and respiratory disturbance than controls. The results of the Conners scale showed that they were between five and ten times more likely to display aggressive, inattentive and hyperactive behaviour patterns that controls when assessed by their parents. A similar pattern was reported by teachers for inattentive and hyperactive but not aggressive behaviour. Children with sleep and breathing disorders were three times more likely to have a mother who smoked. The smoking behaviour of mothers accounted for the excess of snoring in lower socio-economic groups. A subgroup of the children studied at home had overnight urinary growth hormone measurements made to determine whether sleep disordered breathing was associated with reduced growth hormone output. This study found no significant differences between those with and those without sleep and breathing problems in growth hormone output.