The early response to therapy in childhood acute lymphoblastic leukemia (ALL) is typically assessed by bone marrow status. The clearance of blast cells in peripheral blood and bone marrow during induction therapy was analysed in 38 children (27 precursor-B-ALL and 11 precursor T-ALL) treated according to BFM90 or New York 93 protocols. Leukocyte count and peripheral blood smear taken at diagnosis, day 8, day 14 and day 33 as well as bone marrow cellularity and percentage of blasts on days: 0, 14, 33 were analysed. The number of blasts in bone marrow was correlated to bone marrow cellularity (Spearman's rho = 0.72, p = 0.001). Patients with T-ALL were more frequently resistant to steroids in vivo and reached remission later in comparison to precursor-B-ALL children (p = 0.019), however blasts reduction ratio was comparable in both lineages. Bone marrow cellularity on days 14 and 33 of induction therapy was similar.