This study evaluated the relationship of caesarean section to the risk of asthma in adulthood. The data were based on a prospective birth cohort born in northern Finland in 1966. In 1997, when the members of the cohort were 31 years old, information on current doctor-diagnosed asthma and other allergic disorders was obtained from 1953 subjects by a self-administered questionnaire and skin prick test. Caesarean section had a strong effect on current doctor-diagnosed asthma in adulthood with an adjusted odds ratio (OR) of 3.23 (95% CI 1.53, 6.80). However, no substantial effects were observed for atopy, hay fever, and atopic eczema.