A method for the measurement of 6-mercaptopurine (6MP) in urine using high-performance liquid chromatography is described. Urinary excretion of 6MP was measured in 46 children with acute lymphoblastic leukemia. The proportion of unchanged drug excreted after oral dosage in the morning was greater than after an evening dose (5.6 +/- 3.3% vs. 3.3 +/- 2.4%). Possible reasons for the discrepancy are discussed. In all children receiving 6MP in the morning, the drug was detected in urine at 2 and 4 h after ingestion. At 6 h, 6MP was still detectable in 77% of patients, at 8 h in 54%, at 10 h in 12%, and at 12 h in 8%. The reliability of urinary 6MP determination as a measure of drug compliance was assessed in 39 children accustomed to receiving their medication in the evening. 6MP was detected in 81% of first morning urine samples, indicating compliance with medication the preceding evening. The absence of 6MP in first morning urine samples did not necessarily indicate poor compliance because of the variability in 6MP excretion and unpredictable pattern of night voiding in children. The method was therefore a reliable measure of good short-term compliance. It also directed attention toward possible noncompliance in children with negative samples.