Forty-one previously untreated children with Stage IV non-Hodgkin's lymphoma were studied from January 1971 to April 1979. All patients had bulky disease histologically proven to be non-Hodgkin's lymphoma with bone marrow involvement. They were separated into two groups according to the extent of bone marrow involvement. Group IVA included 14 patients with 25% or less lymphoblasts in the bone marrow. Group IVB included 27 patients with more than 25% blasts in the marrow. Their clinical characteristics with regard to age, sex, hemogram, histology, primary site, and blast morphology are compared. All were treated with the LSA2-L2 protocol with radiation therapy to one or more bulky sites of involvement. The disease-free actuarial survival for Group IVA was 64% with a median observation time of 49 months while that for Group IVB was 65% with a median observation time of 66 months. There was no statistical difference in the survival rates between the two groups. Hence, we conclude that the extent of bone marrow involvement does not affect the prognosis in Stage IV non-Hodgkin's lymphoma. It appears that radiation therapy may have contributed to the improved survival in our series. Furthermore, a subset of patients in Group IVB (24/27) who could be considered as high-risk acute lymphoblastic leukemia on the basis of age, initial leukocyte count, hemoglobin, mediastinal mass or T- or B-cell markers showed an improved survival (73% versus 43%) when compared to patients treated with conventional leukemia regimens.