Cytogenetic studies were performed on ML cell lines (ML-1, -2, and -3), as well as on the leukemic cells of a patient from whom the ML cells were derived. The ML-1 cell line showed numerical and structural cytogenetic changes, i.e., -Y, 1p-, 6q-, 11q-, +12, +13q+, 14q-, and 17q-. The ML-2 cell line had two copies of the 13q+, whereas the ML-3 cells contained three clones, i.e., 47,X,-Y,1p-,6q-,11q-,+12,+13q+, 48,X,-Y,1p-,6q-,+6q-,11q-,+12,+13q+, and 49,X,-Y,1p-,6q-,+6q-, 11q-,+12,+13q+,+13q+. The neoplastic cells, when the patient was diagnosed as having T-cell malignant lymphoma (Stage IV), had the 11q- and 13q+. The leukemic cells in a subsequent acute myeloid leukemia phase of this patient contained structural (1p- and 6q-) and numerical (+12, -Y and +2D-group chromosomes: two 13q+) changes in addition to the 11q-. These findings suggest that the acute myeloid leukemic cells of this patient probably originated from the neoplastic cells of the preceding T-cell lymphoma, and that the chromosome changes originally seen in the lymphoma cells were preserved in the established ML cell lines, though the cells of these lines had myeloid characteristics.