In five out of 51 adult, male patients with acute non-lymphocytic leukemia, Klinefelter's syndrome was demonstrated karyotypically. The incidence of this syndrome in our material is very much increased in comparison with the frequency at birth. In this article the medical histories of the patients are given and the possible connection between the constitutional chromosome abnormality and acute non-lymphocytic leukemia is discussed. Immunological or hormonal abnormalities or an increased virus susceptibility might be predisposing factors.