This study was designed to assess the predictive value of an MTT (3-[4,5-dimethylthiazol-2,5-diphenyl] tetrazolium bromide) in vitro assay for the evaluation of leukemic cell resistance/sensitivity to cytotoxic drugs, and to compare these results with clinical and laboratory parameters in cases of childhood acute lymphoblastic leukemia (ALL).The chemoresistance of leukemic cells was ascertained by means of an MTT assay in 32 previously untreated children with ALL. The children were treated using the protocol ALL-BFM 90. Statistical correlations were made between in vitro drug resistance to anti-cancer drugs: prednisolone (PRED), vincristine (VCR),daunorubicin (DNR), etoposide (VP-16) and cytosine arabinoside (ARA-C) and in vivo clinical and laboratory parameters: age, sex, risk factor (RF), leukocytes (WBC)and absolute number of blast cells (BC) at diagnosis (BC0), BC at day 8 (BC8), the percentage of blast cells in bone marrow at day 15 (BM15) and at day 33 (BM33),and leukocyte surface antigens CD3, CD4, CD5, CD8, CD10, CD19, CD20, HLADR.