RATIONALE: Early-life antibiotic use has been associated with development of atopic diseases, but the aetiology remains unclear. To elucidate aetiology, we used a discordant twin design to control for genetic and environmental confounding. METHODS: We conducted a retrospective cohort study in twins (3-10 years) from the Netherlands Twin Register (NTR, n=34 352) and a replication study at age 9 in the Childhood and Adolescent Twin Study in Sweden (CATSS, n=7906). Antibiotic use was recorded at 0-2 years. Doctor diagnosed asthma and eczema were reported by parents when children were 3-12 years in both cohorts. Individuals were included in unmatched analyses and in co-twin control analyses with disease discordant twin pairs. RESULTS: Early-life antibiotic use was associated with increased risk of asthma (NTR OR 1.34 95%CI 1.28-1.41; CATSS 1.45 95%CI 1.34-1.56) and eczema (NTR OR 1.08 95%CI 1.03-1.13; CATSS 1.07 95%CI 1.01-1.14) in unmatched analyses. Co-twin analyses in mono- and dizygotic twin pairs showed similar results for asthma (NTR 1.54 95%CI 1.20-1.98 and CATSS 2.00 95%CI 1.28-3.13), but opposing results for eczema in NTR (0.99 95%CI 0.80-1.25) and CATSS (1.67 95%CI 1.12-2.49). The risk of asthma increased for antibiotics prescribed for respiratory infections (CATSS 1.45 95%CI 1.34-1.56), but not for antibiotics commonly used for urinary tract/skin infections (CATSS 1.02 95%CI 0.88-1.17). CONCLUSION: Children exposed to early-life antibiotic use, particularly prescribed for respiratory infections, may be at higher risk of asthma. This risk can still be observed, when correcting for genetic and environmental factors. Our results could not elucidate whether the relationship between early-life antibiotic use and eczema is confounded by familial and genetic factors.