Essential thrombocythemia (ET) is a myeloproliferative disorder characterized by a platelet count higher than 1000 x 10(9)/l. Bone marrow karyotype aberrations are occasionally observed. The presence of cytogenetic and molecular markers of chronic myeloid leukemia (CML) was assessed in 25 patients with the clinical features of ET. One displayed a complex translocation (9; 15; 22) (q34.1 or q34.3; q26.1; q11), and another a Philadelphia chromosome with standard translocation (9; 22) (q34; q11). Southern blot analysis revealed a rearranged breakpoint cluster region (bcr) in each case. Both patients experienced a stormy disease course without a leukemic transformation. These data indicate that the Philadelphia chromosome rarely occurs in ET and strongly influences patient outcome.