Trisomy 14 was the sole karyotypic anomaly in three patients with Ph1-negative chronic myeloid leukemia, and the only abnormality in one of three clones in a fourth case. The hematologic features were partly myeloproliferative, partly myelodysplastic, and included myeloid hyperplasia, neutrophilia without basophilia, a relatively high number of immature granulocyte precursors in the peripheral blood, and monocytosis in three and dysgranulopoiesis in two of the patients. These data, in combination with the patients' high age at diagnosis, their short survival, and the lack of rearrangements of the major breakpoint cluster region (M-bcr) in the two cases where cells were available for molecular analysis, indicate that all four patients suffered from atypical chronic myeloid leukemia (aCML). We suggest that trisomy 14 may be a characteristic karyotypic abnormality in this hematologic disorder.