The study will investigate, in children with acute lymphoblastic leukemia during maintenance
treatment, if addition of allopurinol to conventional oral 6-mercaptopurine and methotrexate
therapy, affects erythrocyte concentrations of 6-thioguanine and 6 methylmercaptopurine. The
effect on hematological and liver toxicity parameters in blood will also be investigated as
well as clinical toxicity.