Bcr-abl mRNA expression was studied in patients with chronic myeloproliferative disorders (CMPD) by the reverse transcriptase-polymerase chain reaction (RT-PCR) method. A bcr-abl transcript was not found in any patient with polycythemia vera, essential thrombocythemia or primary myelofibrosis, suggesting that the bcr-abl rearranged clone is not present in CMPD other than chronic myelogenous leukemia (CML). In CML clinical and laboratory data were compared from three bcr-abl types: the bcr exon 2-abl exon 2 (B2-A2) type, bcr exon 3-abl exon 2 (B3-A2) type and the co-expression type. Age at diagnosis tended to be younger (p=0.08) in the co-expression type, and the platelet count tended to be lower (p=0.11) in the B2-A2 type. However, there was no difference in other data, including the duration of the chronic phase and the phenotype of blasts at blast crisis.