We studied the karyotype in 81 consecutively diagnosed children with acute lymphocytic leukemia (ALL) treated at one institution on a randomized treatment protocol. In 75 patients (93%), a morphological cytogenetic result was obtained, and 57 (65%) were successfully G-banded. Of the 75 patients, 46 (61%) showed abnormal chromosomes, mainly hyperdiploidy and pseudodiploidy, and 29 had no detectable abnormality. Our findings confirmed that the karyotype has prognostic significance. Duration of complete remission was 93% at 42 months for patients with high hyperdiploidy (greater than 50). For patients with an apparently normal karyotype, it was 58%; and for patients with structural abnormalities it was 15%. The significance of these findings was confirmed by multivariate analysis, which showed age and karyotype to be the most important determinants of duration of remission.