To explore a possible relationship between environmental factors encountered during fetal life and infancy and the later risk of developing Parkinson's disease, a case-control study (172 cases, 343 age- and sex-matched controls) was carried out in 42 general practices in the eastern part of Hertfordshire, England. Information about birthweight and growth during the first year of life was obtained for subjects who had been born in Hertfordshire from records made by health visitors in the first part of the century. Information concerning size of sibship, position in the birth order, type of housing, other features of the environment in early life and experience of the common infections of childhood was obtained by questionnaire. Neither birthweight, weight at 1 year of age nor any aspect of the childhood domestic environment were associated with an altered risk of Parkinson's disease. Cases were more likely to recall suffering from croup (odds ratio 4.1, 95% CI 1.1 to 16.1) or diphtheria (odds ratio 2.3, 95% CI 1.2 to 4.7) in childhood than controls but no other infection was associated with an increased relative risk for Parkinson's disease. Cases were more likely than controls never to have smoked cigarettes (odds ratio 2.0, 95% CI 1.1 to 3.6). The results of this study do not suggest that poor growth in fetal life or infancy is important in the aetiology of Parkinson's disease but they hint that early infection might partly determine susceptibility to the disease.