Certain constitutional chromosomal abnormalities increase the risk of malignancy and/or decrease treatment tolerance. We identified two patients with the XYY syndrome among a total of 444 male children with acute lymphoblastic leukemia who had complete cytogenetics studies. In both cases, the leukemic cell karyotype suggested a constitutional XYY abnormality that was confirmed in studies of lymphocytes obtained during remission. The incidence rate in our series is higher than that of the XYY syndrome in the general population (0.0045 vs. 0.001), but not significantly so. This finding and a literature review failed to confirm an increased frequency of the XYY syndrome among children with acute lymphoblastic leukemia. Both of our patients remain in remission 24 and 28 months, respectively, postdiagnosis. Their tolerance of intensive treatment, including high-dose methotrexate, suggests that the untoward treatment toxicity seen in patients with chromosomal abnormalities such as trisomy 21 does not extend to the XYY syndrome.