A 35-year-old pregnant woman visited our outpatient clinical questioning the safety of once daily 50 mg mercaptopurine (MP) during pregnancy and lactation, which was successfully treating her Crohn's disease. We measured MP and its metabolites in plasma and breast milk and found after 4 hours of intake of MP, no MP or its metabolites in breast milk. We concluded that 4 hours after intake of MP, no exposure of the suckling infant to MP and its metabolites was found while being breastfed.