Cytogenetic analyses were performed on 12 adult patients with abnormal megakaryoblastic proliferation which was detected by ultrastructural cytochemical study (platelet peroxidase) and platelet-megakaryocytes-specific monoclonal antibodies (TP-80, Plt1, AN51, and KOR-77). The patients consisted of two patients with myelodysplastic syndromes (MDS), three with acute megakaryoblastic leukemia (AMKL), six with megakaryoblastic transformation in Philadelphia-positive chronic myelogenous leukemia (CML-meg-BC), and one case of chronic myeloproliferative disorder (CMPD). Among them, an inversion of the long arm of chromosome 3 [inv(3)(q21q26)] was found in one AMKL patient with a normal platelet count. Chromosome change at band 3q26 was also found in one MDS patient without thrombocythemia. Furthermore, the long arm of chromosome 13, where rearrangements in myelofibrosis are clustered (13q12----q22) was seen in one MDS patient. Trisomoy of chromosome 19 was found in one AMKL patient and three CML-meg-BC patients. These findings indicate that cytogenetic abnormalities involving 3q26, 13q, and trisomy 19 are associated with hematologic neoplasia with megakaryocytic lineage in adult patients, although these abnormalities were not related to the survival of the patients. During the period of this study, two acute myelogenous leukemia patients (AML-M2 and AML-M5b) with chromosome rearrangements at band 3q21 and thrombocythemia were found, indicating that chromosome abnormality at band 3q21 is related to quantitative platelet dysfunction, whereas that at 3q26 is related to hematologic malignancies with a proliferation of megakaryocytic lineage.