Traditionally, women receiving azathioprine have been discouraged from breastfeeding because of theoretical potential risks of neonatal bone marrow suppression, susceptibility to infection, and pancreatitis. The aims of this study were to measure the concentration of 6-mercaptopurine (6-MP) in breast milk of mothers receiving azathioprine and in the blood of their babies and to investigate any immunosuppressive effects on the babies. Women receiving azathioprine, who after appropriate counselling wished to breastfeed their babies, were approached for inclusion in the study. Breast milk samples were obtained from recruited women, and 6-MP levels were measured in each breast milk sample. Haemoglobin level, white cell and platelet counts, and 6-MP and 6-thioguanine nucleotides (6-TGN) levels were measured in the respective neonatal blood samples. Clinical signs of immunosuppression in the neonates were noted. Thirty-one breast milk samples were collected from ten women. Low concentrations of 6-MP (1.2 and 7.6 nanograms/ml, compared with therapeutic immunosuppressant level of 50 nanograms/ml in serum) were detected in two breast milk samples obtained from one woman. 6-MP was not detected in any of the other 29 samples. 6-MP and 6-TGN were undetectable in the neonatal blood. There were no clinical or haematological signs of immunosuppression in any of the ten neonates. We conclude that breastfeeding should not be withheld in infants of mothers receiving azathioprine.