In 48 patients with acute non-lymphoblastic leukemia, all of whom had been treated according to the protocol 7421 of the "acute leukemia group B", remission rates and survival times were correlated with the chromosome constitution of bone marrow cells at diagnosis. 45.8% of the patients had only normal metaphases (N-patients), 31.3% had normal and abnormal metaphases (AN-patients), and 22.9% had only abnormal metaphases (AA-patients). Chromosomal findings were unrelated to patients' age. The remission rate of the N-patients was 72.7%, of the AN-patients 60%, and of the AA-patients 36.4%. The respective median survival times were 12.5, 8.5 and 4.0 months. The difference in remission rates and survival times between patients with normal and without normal metaphases was significant. Once a remission had been obtained the prognosis was similar among the 3 groups. The better prognosis of the AA-patients in this study as compared to previous reports might be related to a more effective chemotherapy.