To pregnant or breast feeding women drugs should be given with caution. We report the case of a 5-week-old breast-fed infant whose mother was taking 300mg allopurinol/day for 4 weeks. Allopurinol and oxypurinol were detected by HPLC in maternal plasma and breast milk with a method first described here. In infant's plasma taken 2 h after breast feeding oxypurinol was found; allopurinol was below the limit of detection. The milk/plasma ratio in the mother 2 h (4 h) after drug ingestion was 0.9 (1.4) for allopurinol and 3.9 (2.4) for oxypurinol. The average daily dose for the baby of allopurinol was 0.14-0.20 mg/kg and that of oxypurinol 7.2-8.0 mg/kg by ingestion of breast milk after oral intake of allopurinol by the mother.