The major bcr-abl fusion gene is seen as a major marker of chronic myeloid leukemia (CML). However, whether the bcr-abl transcript can be detected in patients with essential thrombocythemia (ET) is still a matter of controversy. We detected the messenger RNA expression of the bcr-abl gene using reverse transcription-polymerase chain reaction in peripheral-blood leukocytes (PBLs) from 63 patients with myeloproliferative disorders (including CML, ET, and polycythemia vera [PV]) and 51 normal, healthy volunteers. The bcr-abl transcript was detected in 4 of the 30 ET patients (13.3%), 17 of the 17 CML patients (100%), none of the 16 PV patients (0%), and 1 of the 51 normal subjects (1.9%). Compared with the normal controls, ET patients have a greater tendency to express the bcr-abl transcript in PBLs (P=.06, Fisher's exact test). Further semiquantitative analysis showed that the intensity of bcr-abl transcript expression in 4 ET patients and a normal individual was 10(3) to 10(4) times less than that in the CML patients. We conclude that the bcr-abl transcript can be detected in the PBLs of Philadelphia chromosome (Ph)-negative ET patients but that the level of expression is markedly less than that in CML patients. The clinical significance of this finding merits further investigation.