The effect of the underlying disease and chemotherapy on megakaryopoiesis has not been extensively studied in children with acute lymphoblastic leukemia (ALL) during and at the end of therapy. Using a serum-free assay, we assessed the megakaryocyte (Mk) colony formation in vitro from bone marrow mononuclear cells of 25 children with ALL during chemotherapy and shortly after the cessation of it. Twelve children with solid tumors without bone marrow involvement and cord blood from 10 full-term normal vaginal deliveries were used as controls. A significant reduction in the number of Mk colonies was observed at diagnosis of ALL, and Mk colony formation remained lower than controls throughout the different phases of leukemia treatment. Our study suggests that defects in megakaryopoiesis of children with ALL in long-term remission may persist during chemotherapy and at least shortly after the end of it.