To better understand the Philadelphia-negative (Ph-) chronic myeloid leukemia (CML) and its relationships with Philadelphia-positive (Ph+) CML and chronic myelomonocytic leukemia (CMML), a study was undertaken by the Groupe Francais de Cytogenetique Hematologique. Thirty-five Ph- CML patients were investigated and compared with 55 chronic phase Ph+ CML and 100 CMML patients. There were 12 M-BCR positive (M-BCR+) and 23 M-BCR negative (MBCR+) patients. No clinical or biologic differences were found between Ph+ and Ph-, M-BCR+ patients. In the Ph- group, M- BCR+ and M-BCR- patients differed significantly in age (47.7 +/- 6.6 v 67.0 +/- 6.1 years, respectively; P = .001), leukocytosis (153.4 +/- 135.1 v 58.5 +/- 37.7 10(9)/L, P = .002), relative monocytosis (1.8% +/- 1.2% v 5.6% +/- 1.4%, P = .048), absolute basophilia (8.5 +/- 9.7 v 0.9 +/- 1.5 10(9)/L, P = .001), percentage of immature myeloid precursors (IMP) in peripheral blood (29.0% +/- 9.5% v 15.3% +/- 8.1%, P = .001), and percentage of erythroblasts in bone marrow (BM) (6.5% +/- 3.5% v 14.6% +/- 3.6%, P = .001). Karyotypic abnormalities other than the Ph chromosome occurred in 0 of 12 M-BCR- at diagnosis and 7 of 23 M-BCR- Ph- CML (P = .033). None of the 13 investigated BCR- patients had detectable BCR/ABL transcripts using polymerase chain reaction (PCR) and none had an N-RAS mutation. Cytologic findings showed a marked morphologic difference between M-BCR+ and M-BCR- patients, especially in the monocytic lineage. Dysmyelopoietic features in CMML and M-BCR- patients were very similar, and the differences were of quantitative order only. Using four criteria (monocytosis, percentage of IMP, basophilia, and percentage of erythroblasts in BM), patients could be divided into typical and atypical CML and this classification correlated well with molecular findings. We conclude that, while Ph-, M- BCR+, and Ph+ CML are identical diseases, Ph-, M-BCR- CML, and CMML have many similarities and might be only different aspects of a same entity.