We examined in vivo spontaneous and prednisolone-induced apoptosis in peripheral blood samples of 23 children with ALL by flow cytometric and morphologic methods. There was no significant spontaneous apoptosis before the therapy. Six hours after prednisolone therapy, increased apoptosis was found in 19 of 23 cases. In one case, the apoptosis of blast cells could not be compared with the clinical data, because the patient died in sepsis during the induction therapy. In 18 of 22 evaluable cases, the in vivo apoptosis correlated with the decrease of leukemic blasts during the first 8 days of prednisolone monotherapy. In 20 of 22 children, a correlation was found between in vivo prednisolone-induced apoptosis and clinical outcome. The p53 gene expression was elevated in 2 of 10 patients. No elevation in the expression of bcl-2 gene was observed. In 6 of 23 cases, the glucocorticoid receptors were measured. The correlation of clinical responsiveness and gcR mRNA shows less parallelism than does the apoptosis correlation.