Seven years after hospitalisation for acute lower respiratory tract infection in infancy, 200 children and their case -controls were assessed for respiratory status and epidemiological characteristics. The index group comprised 100 cases where respiratory syncytial virus had been responsible for the index illness (RSV +) and 100 cases in whom this organism had not been found (RSV -). No differences were noted between index and control children with respect to birth weight and gestational age, although breast feeding was more frequently observed in control children. Index children reported more respiratory symptoms and asthma as well as other indices of respiratory illhealth. Although index children appeared to be a socially disadvantaged group, parental respiratory symptoms and smoking habits were comparable in both groups of children. The atopic background was similar in index and control populations. At follow -up, index children were shorter than controls, although their weights were comparable. Tests of respiratory function were diminished in index children, who also had evidence of bronchial hyperreactivity. RSV+ and RSV- index children showed similar clinical characteristics and atopic background. No significant differences were found in the age at which the index illness occurred, or in the proportion who were breast -fed. The results of tests of respiratory function and exercise test were comparable in RSV+ and RSV- children. Children who had suffered bronchitis, bronchiolitis or pneumonia had similar clinical characteristics compared to their controls, with the exception that fewer children who had bronchiolitis were breast -fed, and children who suffered pneumonia were of lower birth weight. All three sub -groups of index children reported more respiratory symptoms and ill health than their controls. Social and family factors were less favourable when compared to control children. The atopic background was similar between the three disease categories, and also between index and control children. Tests of respiratory function were significantly reduced only in children who had bronchiolitis, although the trend in the bronchitis and pneumonia children was also towards poorer function. A one -way analysis of variance between disease categories on the differences between case and control for each respiratory function measurement showed that differences within a disease category was geater than the differences between disease categories. Following the index illness, children were reported to cough, wheeze or remain asymptomatic. Those with symptoms were almost identical in terms of clinical, social and family characteristics, as well as atopic background; but differed from their controls. Tests of respiratory function were diminished in both groups of symptomatic children, with evidence of bronchial hyper- reactivity. The asymptomatic index children did not differ from the symptomatic children with respect to social factors, suggesting that these contribute little to the occurrence of respiratory symptoms. Asymptomatic children were of similar height to their controls. Respiratory function was also comparable although there was a slight trend to hyper-reactive airways. Children whose index illness was attributed solely to acute infections (RSV +, non -atopic) reported similar occurrences of respiratory symptoms to their controls. Respiratory function was also comparable. When there was a background history of atopy, children reported more respiratory symptoms and had significantly lower tests of respiratory function as well as evidence of bronchial hyperreactivity when compared with controls. The results suggest that atopy is a determinant of poor respiratory function, but they may have also been influenced by the discrepancy in numbers. Bronchial reactivity was present in excess in atopic and non -atopic index cases, despite being significant only in the atopic children. Index children with bronchial reactivity showed similar clinical, atopic, social and family characteristics to those without evidence of hyperreactive airways, but these two groups differed clinically and in social and family background from control children. Those with bronchial reactivity wheezed more, and there was a greater percentage of asthmatics. Respiratory function was significantly diminished compared to controls. There was no excess of atopic disorders in the index children with hyperreactive airways. Children without evidence of hyperreactive airways also reported more respiratory symptoms, but bronchitis rather than asthma was diagnosed in these children. Except for a lower PEFR, all other tests of respiratory function were similar between index and control children. Ventilatory dysfunction paralleled bronchial reactivity. It is not clear which is of primary importance, or if there is any relationship between the two. Acute respiratory infection may have caused both these abnormalities, or they could have predated the event, rendering children more susceptible to infection.