Plasma levels of 6-mercaptopurine were determined in 22 consecutive children with acute lymphoblastic leukemia on oral remission maintenance therapy during the time period of August 1984 to January 1988. Each child received the drug once daily for up to 3 years and was studied repeatedly (1–12 times). An HPLC method was used for drug analysis. We found large interpatient variations in the mean peak plasma concentration (range of 50–424 ng/ml) and in the mean area under the concentration vs. time from 0–4 h curve (range of 82–637 ng ml-1h). There were also pronounced variations between different sampling occasions in the same patient. Nine of the 22 patients had complications during the maintenance therapy. Five children with a mean peak plasma level below 135 ng/ml and a mean area under the curve (AUC) value below 251 ng ml-1h relapsed (three in the central nervous system and two in the bone marrow). Both children with a bone marrow relapse died. Relapse risk was related to the AUC (p< 0.05). Four children with a mean peak plasma level above 166 ng/ml and a mean AUC value above 363 ng/ml/h developed severe myelotoxicity, which necessitated a temporary cessation of the maintenance therapy. In addition, two patients relapsed 6 and 11 months after termination of maintenance therapy. Their mean peak and AUC values were not low but the concentrations decreased markedly towards the end of the maintenance period. The results indicate that the plasma levels of 6-mercaptopurine, when determined repeatedly, might be of significance for the outcome of the remission maintenance treatment.