Sister chromatid exchange (SCE) was studied in PHA-stimulated peripheral blood lymphocytes from 36 newly diagnosed and untreated leukemic patients: 16 with acute lymphoblastic leukemia (ALL), 10 with acute nonlymphocytic leukemia (ANLL), and 10 with chronic myelocytic leukemia (CML). The metaphases analyzed show no chromosomal abnormalities. The mean SCE frequency (mean +/- SE) for each group of patients was: 6.8 +/- 0.4, 6.6 +/- 0.3, and 7.0 +/- 0.6 per mitosis, respectively, which was significantly lower than the mean SCE score for 30 controls (8.7 +/- 0.2). No differences in SCE score among ALL, ANLL, and CML and a similar SCE frequency by chromosome number and group allowed consolidation of all the cases into a single group of 36 leukemic patients (6.8 +/- 0.3). When the frequency of SCE was compared by chromosome number and group between the leukemic patients with the control group, a significant decrease in SCE frequency was observed due to a low SCE score in almost all the complements, except chromosome #1. It is suggested that the low SCE rate is related to the leukemic process itself.