In the treatment of acute leukaemia in 12 adult patients, an initial course of 6-mercaptopurine (6-MP) 500 mg/day for 5 days was given, later reducing the maintenance dosage to tolerance. A complete remission could not be achieved in any case, but a partial remission resulted in 50% of the cases. The median survival period was only 50 days. It is likely that the results of 6-PM treatment for acute leukaemia in adults cannot be improved by a higher dosage. As more effective combinations of drugs are now available for induction treatment of acute leukaemia, the role of 6-PM seems to be centred on the maintenance of the remissions.