To assess the prognostic value of the cellular DNA content measured by flow cytometry in acute lymphoblastic leukemia (ALL), we studied the pretreatment distribution of the DNA content in marrow blasts from 74 children with this disease. They were divided into standard- and high-risk groups according to their white blood cell counts and age at the time of diagnosis and were followed for up to 44 months. Abnormal DNA stemlines were detected in 26 children, and all of them had a hyperdiploid DNA content (DNA index of greater than 1.0). The duration of remission was significantly longer in those with hyperdiploid DNA stemlines than in those with diploid DNA stemlines in both risk groups. In the standard-risk group, the DNA content and common ALL antigen were significant prognostic factors, especially the former. In the high-risk group, the DNA content also showed an independent significant correlation with the clinical outcome.