BACKGROUND Recommendations on breastfeeding under thiopurines are inconsistent due to limited data. OBJECTIVE To assess the risk of infections in offspring breastfed by mothers receiving azathioprine (AZA) for inflammatory bowel disease (IBD). METHODS Babies, who were breastfed from their mothers treated either with or without AZA were included from a local pregnancy-registry. Women were asked by structured personal interview on general development, infections, hospitalisations and vaccinations of their offspring. RESULTS A group of 11 mothers taking AZA (median 150 mg/d) during pregnancy and lactation and another of 12 patients without using any immunosuppressive therapy breastfed 15 babies each for median 6 months and 8 months, respectively. Median age of children at time of interview was 3.3 and 4.7 years, respectively. All offspring showed age-appropriate mental and physical development. Infections were commonly seen childhood diseases. Similar rates were observed for most of the various infections between offspring with and without azathioprine exposure during breastfeeding. However, common cold more than two episodes/year and conjunctivitis were numerically more often reported in the group without AZA exposure. In an exploratory analysis no difference in the rate of hospitalisations was seen between exposed (0.06 hospitalisations/patient year) versus non-exposed children (0.12 hospitalisations/patient year, p=0.8) CONCLUSIONS Our study which reports the largest number of babies breastfed with exposure to AZA suggests that breastfeeding does not increase the risk of infections.