To determine whether childhood antianaerobic antibiotic exposure is associated with the development of inflammatory bowel disease (IBD). This retrospective cohort study employed data from 464 UK ambulatory practices participating in The Health Improvement Network. All children with ≥ 2 years of follow-up from 1994 to 2009 were followed between practice enrollment and IBD development, practice deregistration, 19 years of age, or death; those with previous IBD were excluded. All antibiotic prescriptions were captured. Antianaerobic antibiotic agents were defined as penicillin, amoxicillin, ampicillin, penicillin/β-lactamase inhibitor combinations, tetracyclines, clindamycin, metronidazole, cefoxitin, carbapenems, and oral vancomycin. A total of 1072426 subjects contributed 6.6 million person-years of follow-up; 748 developed IBD. IBD incidence rates among antianaerobic antibiotic unexposed and exposed subjects were 0.83 and 1.52/10000 person-years, respectively, for an 84% relative risk increase. Exposure throughout childhood was associated with developing IBD, but this relationship decreased with increasing age at exposure. Exposure before 1 year of age had an adjusted hazard ratio of 5.51 (95% confidence interval CI: 1.66-18.28) but decreased to 2.62 (95% CI: 1.61-4.25) and 1.57 (95% CI: 1.35-1.84) by 5 and 15 years, respectively. Each antibiotic course increased the IBD hazard by 6% (4%-8%). A dose-response effect existed, with receipt of >2 antibiotic courses more highly associated with IBD development than receipt of 1 to 2 courses, with adjusted hazard ratios of 4.77 (95% CI: 2.13-10.68) versus 3.33 (95% CI: 1.69-6.58). Childhood antianaerobic antibiotic exposure is associated with IBD development.