The etiology of allergic disease is not understood, but a decreased exposure to infection may play an important role. There are few published data on the impact of change in microbial exposure during pregnancy on the child's risk of developing allergic disease. Using a birth cohort of 24,690 children, derived from the West Midlands General Practice Research Database, we investigated a number of perinatal exposures on the incidence of asthma, eczema, and hay fever. Our findings suggest that exposure to antibiotics in utero is associated with an increased risk of asthma in a dose-related manner (more than two courses of antibiotics compared with none adjusted hazard ratio [HR] 1.68; 95% confidence interval [CI], 1.51-1.87), and similar associations are present for eczema (adjusted HR 1.17; 95% CI, 1.06-1.29) and hay fever (adjusted HR 1.56; 95% CI, 1.22-2.01). Exposure to a range of infections in utero was also associated with a small increased risk of developing allergic disease. Strong protective effects of older siblings on the incidence of allergy are present within this cohort, but previous pregnancies that did not result in a live birth were not protective. Our findings suggest that exposure to antibiotics and to infections in utero is a potentially important risk factor in the development of allergic disease.