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Treatment of children with acute lymphoblastic leukemia (ALL) is based on P-glycoprotein (P-gp)-dependent cytostatics. We assessed the P-gp function in blast cells as a possible prognostic factor and its influence on the overall survival. P-gp function was measured using the verapamil-sensitive Rhodamine efflux. Cell samples from 7 of 45 (16%) patients revealed rhodamine-efflux positive blasts. There were no relations between the presence of P-gp, clinical characteristics (age, sex, hepatomegaly, and splenomegaly) and initial laboratory parameters (immunophenotype, white blood cells count, and serum lactate dehydrogenase) in ALL. P-gp activity plays a negative role, both for a remission achieved on day 33 and for susceptibility to steroid therapy. Children bearing rhodamine-efflux positive blasts had a significantly shorter 5-year overall survival of 35%, as compared with 74% in those negative for P-gp function. Lack of any association with clinical characteristic and initial laboratory parameters suggests that presence of P-gp is an independent prognostic factor.