Sensitivity of leukemic blasts to steroid therapy is one of the prognostic factors in acute lymphoblastic leukemia (ALL) in children. We examined the number of steroid receptors and the increase in the apoptotic index in peripheral blast cells after administration of prednisolone monotherapy in 21 children with ALL. A new diagnostic method was established based on determination of the apoptotic index in peripheral blood lymphoblasts to evaluate the steroid sensitivity of leukemic cells during the first day of therapy. The increase in apoptotic ratio, analyzed by morphologic and/or flow cytometric studies, was most expressed in the first 6 hours of treatment. The apoptotic ratio showed a good correlation with the clinical response. The number of steroid receptors (gcRs) on the blast cells was also examined, but it proved to be less informative than the in vivo steroid response itself.