Structural chromosomal aberrations have been described in various types of human leukemia. The micronucleus technique provides a measure of both chromosome breakage and chromosome loss. The present study investigated micronucleus (MN) frequency in mitogen-stimulated peripheral blood lymphocytes from 20 newly diagnosed and untreated leukemia patients: 4 with acute lymphoblastic leukemia (ALL), 10 with acute myeloid leukemia (AML), and 6 with chronic lymphocytic leukemia (CLL). The mean MN frequency for untreated patients was 3.65 +/- 1.47 in ALL, 3.55 +/- 1.24 in AML, 3.03 +/- 1.05 in CLL. No differences in MN frequency were seen between leukemia types ALL, AML, and CLL (P = 0.503). The mean basal MN frequency for all patients, regardless of leukemia type, was 3.41 +/- 1.19, which was significantly higher (P = 0.001) than that of 20 age-matched control subjects, 1.87 +/- 0.75. Although no significant relationship was found between age and MN frequency in patients with leukemia (r = 0.050; P = 0.835), the MN frequency in the lymphocytes of healthy control increased regularly and significantly with age (r- = 0.531; P = 0.016). These data indicate that the increased baseline MN frequency in lymphocytes of untreated patients with leukemia may reflect genomic instability or deficiency of DNA repair capacity. MN enhancement in this disease may thus be a consequence of the disease process. (C) 2008 Elsevier Inc. All rights reserved.