A statistical analysis of possible risk factors for relapse during maintenance therapy (MT) for acute lymphoblastic leukemia (ALL) has been performed. The patient material consists of 64 patients. Twenty-six patients were classified as standard risk (SR), 21 as intermediate risk (IR), and 17 as high-risk (HR) patients. Seventeen patients relapsed and 50 patients (78%) are alive at a median observation period of 86 months (range 39-146 months). Mean white blood cell count (mean WBC) based on weekly determinations, duration of treatment interruption, and the number of infectious episodes were calculated in each patient during the first 6 months of MT. In analyses starting at 6 months after the beginning of MT, these factors were related to relapse risk, time to relapse, and time to infection. Using the median WBC value (3.9 x 10(9)/L) of all patients during the first 6 months as a cutoff point, 14 patients with levels higher, and 3 patients with levels lower relapsed (p = .0004). Adjustment of mean WBC for leukemia risk groups had no influence on the analysis. Time to relapse was related to duration of interruption of MT (p less than .01). Time to relapse was not related to leukemia risk groups. Infection frequency was higher in HR patients compared to SR and IR risk patients (p = .04). As WBC level had a prognostic value and was previously shown to be related to 6-mercaptopurine (6-MP) peak plasma concentration, monitoring 6-MP plasma levels during MT could be helpful for optimizing treatment.