We report a case of blast crisis in chronic myelogenous leukemia (CML) in which T lymphocytic and megakaryocytic lineages were both involved. A 55-year-old male was initially admitted to Ehime University Hospital because of generalized lymphadenopathy. The morphological features of peripheral blood and bone marrow were consistent with chronic phase of CML. Cytogenetic studies of bone marrow and lymph node cells both showed the Ph1 chromosome with additional abnormalities. The patient was diagnosed as being in the extramedullary blast crisis of CML involving lymph nodes. After six months, blasts increased in bone marrow and peripheral blood. The phenotypes of lymph node blasts were positive for CD2, CD7 and TdT, but negative for CDw41 (platelet glycoprotein IIb/IIIa). On the other hand, those of peripheral blood blasts were positive for CDw41, but negative for CD2, CD7 and TdT. Chromosome studies of lymph node cells and bone marrow cells revealed 46, XY, inv(7) (p15q34), t(9;22) (q34;q11) and 46, XY, t(1;3) (q23:q21), t(9;22) (q34;q11), respectively. The rearrangement of T cell receptor beta chain gene was detected in lymph node blasts, but not in peripheral blood blasts. The identity of the rearrangement patterns of the breakpoint cluster region on chromosome 22 was detected in these blasts. According to these data, it was suggested that blast crisis of CML occurred in two distinct lineages, T lymphocyte and megakaryocyte.