Genetic factors cannot explain the recent rapid increase in the incidence of atopic diseases. The phenomenon has been explained by environmental factors, and there are data for and against the hypothesis that a decline in the pressure of microbial stimulation early in life could be behind the allergy epidemic. Changes have also occurred in maternity care, among them a rise in the caesarean section rate, which could diminish initial microbial exposure and thereby alter T helper 1 cell/T helper-2 cell development and affect the risk of developing atopy. In this study, we sought to establish whether mode of delivery does influence the development of atopic asthma. Finnish 1987 Medical Birth Register (n = 59,927 live births) information was linked between several national health registers to obtain information on asthma and mode of delivery in children registered. The data were adjusted for maternal age, previous deliveries, child's sex, and birth size. Atopy was evaluated in the second study (Turku Birth Cohort), which involved 219 children born by vaginal delivery (n = 106) or caesarean section (n = 113); history of atopic symptoms was established by questionnaire and a clinical examination was conducted, including skin prick testing and determination of total and allergen-specific IgE in serum. The register study showed the cumulative incidence of asthma at the age of seven to be significantly higher in children born by caesarean section (4.2%) than in those vaginally delivered (3.3%), the adjusted odds ratio (OR) for confounding variables being 1.21 (1.08-1.36), p < 0.01. In the second study, significantly more positive allergy tests were reported in questionnaires in the caesarean (22%) than in the vaginal delivery group (11%), OR 2.22 (1.06-4.64), p < 0.01, and a trend toward more positive skin prick reactions was documented at clinical examination; 41% versus 29%, OR 1.31 (0.65-2.65), p = 0.11. In conclusion, these results suggest that caesarean section delivery may be associated with an increased prevalence of atopic asthma.