The bone marrow chromosomes in six ANLL patients with a missing Y were examined. In all of six cases, the missing Y was the only chromosomal abnormality at diagnosis, and it persisted through their clinical courses. One patient had a shift from the 45,X to a 47,XX, +5, 13q + karyotype in the terminal phase of the disease. The findings indicate that the 45,X cells were not involved in further karyotypic progression, and that the leukemic process usually seemed to involve the 46,XY cell rather than the 45,X cells. Based on the data of the current study and that in the literature, it appears that the presence of missing Y in ANLL may afford a rather benign clinical course to such patients when compared to that of the ANLL patients with a missing Y. It is suggested that the age of the patients, karyotypic differences and degree of a missing Y in the marrow cells may affect the clinical course of ANLL.