In a retrospective population-based study of 122 children with non-B-cell acute lymphoblastic leukemia (ALL), we analyzed the relation between risk of relapse and the degree of leukopenia achieved during oral methotrexate (MTX) and 6-mercaptopurine (6MP) maintenance chemotherapy (MT). After a median follow-up of 62 months for patients still in remission, 43 patients had relapsed (including 28 bone marrow relapses). Patients with a mean white blood cell count during MT (mWBCMT) of less than or equal to 3.5 x 10(9)/L had a significantly lower risk of hematological relapse (p = 0.007) as well as of any relapse (p = 0.02) compared to patients with higher mWBCMT. The clinical advantage of leukopenia could be demonstrated for all risk groups and was not explained by differences in year of diagnosis, gender, age, and white blood cell count at diagnosis, or the prescribed dose of MTX and 6MP. Although prospective studies are needed to establish the benefit of upward dose adjustments to achieve leukopenia, these results indicate a clinical advantage of keeping WBCs low during MT.