Five retrospective studies have reported an association between antibiotic use in early life and asthma in childhood. We studied the relationship between the use of oral antibiotics in the first year of life and asthma, allergic rhinitis, and eczema at age 5 years among 448 children with a parental history of atopy monitored from birth. After adjustment for potential confounders, we found no significant association between antibiotic use in the first year of life and asthma (odds ratio [OR] for one versus no courses of antibiotics, 0.5; 95% confidence interval [CI] for OR, 0.2 to 1.5; OR for two or more versus no courses of antibiotics, 1.0; 95% CI for OR, 0.5 to 2.2), recurrent wheezing, allergic rhinitis, or eczema at age 5 years. There was no significant association between antibiotic use in the first year of life and having at least one of three atopic diseases (asthma, allergic rhinitis, or eczema) at age 5 years (OR for one versus no courses of antibiotics, 0.7; 95% CI, 0.4 to 1.4; OR for two or more versus no courses of antibiotics, 0.9; 95% CI, 0.5 to 1.4). Our findings do not support the hypothesis that antibiotic use in early life is associated with the subsequent development of asthma and atopy in childhood.