Using flow cytometric techniques, we determined the pretreatment distribution of DNA content in propidium iodide-stained leukemic blasts from 205 children with 'standard-risk' acute lymphoblastic leukemia (ALL). Risk assignment was based on an initial WBC count <100 x 109/L, no thymic mass, no meningeal leukemia, and lymphoblasts lacking sheep erythrocyte receptors or surface immunoglobulin. A single aneuploid leukemic line was detected in 74 cases (36.1%): 70 hyperdiploid and four hypodiploid. For hyperdiploid cases, the DNA index (DI, or ratio of the DNA content of leukemic v normal G0/G1 cells) ranged from 1.06 to 2.0 (median, 1.20). A secondary leukemic line with hyperdiploid cellular DNA content was identified in 21 cases with diploid primary lines. Children whose primary leukemic line showed a DI ≥ 1.16 (n = 57) had significantly better response to treatment than did those with either a diploid DI (n = 130: P = .002) or values in the range of 1.01 to 1.15 (n = 14; P = .001). The relative risk of failure for hyperdiploid cases with DI ≥ 1.16, corresponding to ≥53 chromosomes, was one-third that of the other two groups. Treatment responses of patients with both diploid and hyperdiploid lines were identical to those associated with single diploid lines, but significantly worse than those associated with single hyperdiploid lines with DI ≥ 1.16 (P = .016). The most favorable prognostic variables selected by a Cox proportional hazards model were: DI ≥ 1.16 (P = .001), white race (P = .022), WBC ≤ 25 x 109/L (P = .032), age between 2 and 9 years (P = .075), and hemoglobin <7.0 g/dL (P = .094). DNA index ≤1.16 retained its significant prognostic impact even after adjustment for other variables (P = .001). With the combination of DI ≥ 1.16 and WBC ≤ 25 x 109/L, one can identify a group of children with ALL who have a low probability of relapse when treated with current therapy. If they remain disease-free after longer follow-up, it may be advisable to treat them with less intensive, hence less toxic, chemotherapy than patients with higher WBC counts or lower DI values.