Rates of immune-mediated diseases (IMDs) have rapidly increased. Although the exact etiology has not yet been fully elucidated, disruptions to the microbiome has been proposed as a potential mechanism. We conducted a retrospective, longitudinal, birth cohort study utilizing electronic health records (EHR) to investigate the association between early life antibiotic exposure and the risk of developing juvenile idiopathic arthritis (JIA), pediatric psoriasis, or type 1 diabetes. Incident rate ratios (IRR) were estimated using modified Poisson regression models and adjusted for significant confounders. Children exposed to two or more antibiotics prior to 12 months of age had a 69% increased risk of developing JIA (1.69 IRR, 95% CI [1.04-2.73]), which rose to 97% when exposed prior to 6 months (1.97 IRR, 95% CI [1.11-3.49]). Children exposed to a penicillin antibiotic had a 62% increase in risk for psoriasis (1.62 IRR, 95% CI [1.06-2.49]), which rose slightly to 64% when exposure occurred between 6 and 12 months of age [(1.64 IRR, 95% CI [1.04-2.59]). We found a moderate to strong association between early antibiotic exposure and risk for JIA and psoriasis when exposure was examined by age, frequency, and type of antibiotic, but not for type 1 diabetes. Potential interactions effects between infection and antibiotics with an increased susceptibility to early life infections among children with an IMD was also observed. Overall, children exposed to antibiotics at an early age have an increased probability of developing an IMD after 12 months of age. However, alternative explanations for this association should be considered. / Public Health