Among 84 consecutive patients with chronic phase Ph-positive chronic myeloid leukemia (CML) who were investigated for the hybrid BCR/ABL mRNA, in six cases (7%) the disease mimicked essential thrombocythemia (ET) at presentation, because of marked thrombocytosis (platelet counts ranging from 1003 x 10(9)/l to 2800 x 10(9)/l) and moderate leukocytosis (WBC counts from 10 x 10(9)/l to 19 x 10(9)/l). At initial examination, four of the six patients showed a few (4-9%) immature myeloid cells in the peripheral blood, and two had blood basophilia. All six patients later developed increasing leukocytosis, and two subsequently died from blast crisis and accelerated CML, respectively. In the overall series, 38 patients (45%) expressed the b2a2 type of BCR/ABL mRNA, and 46 (55%) either the b3a2 or both mRNAs. By contrast, only one of the six patients with CML of thrombocythemic onset expressed the b2a2 mRNA vs five with either b3a2 (n = 4) or both mRNA types (n = 1). The above results, in conjunction with similar data from a few previously published cases, suggest an association between the above form of CML and b3a2 type of BCR/ABL transcript.