Between 1971 and 1981, 699 children were diagnosed to have acute lymphoblastic leukemia (ALL) in Hungary. 34 of these children had received prednisolone therapy prior to the establishment of the diagnosis. The most frequent presumptive diagnoses that prompted steroid treatment were aplastic conditions and arthritic disorders. Leukemia was diagnosed when the presenting symptoms reappeared usually several weeks after the initiation of steroid therapy and often following withdrawal of the drug. Initial leukemic burden, as judged by leukocyte count and hepatosplenomegaly, was smaller in these patients than in other children with leukemia at the time of diagnosis. Although they entered remission at the same rate as the other patients, the length of continuous complete remission was significantly shorter in the prednisolone pretreated group. It appears that prolonged prednisolone therapy given before remission induction imparts a distinct unfavorable prognosis.