We describe a unique case of a young girl with adult type chronic myelocytic leukemia (CML). Clinically she had an extramedullary blastic crisis (BC) prior to that in peripheral blood and in bone marrow. The blasts were primitive cells and always CD7, CD33, CD34 and HLA DR-positive. During the course of illness the blasts became negative for CD5 and positive for CD41a and for platelet-peroxidase. Additional chromosomal appearances and subsequently clonal evolution were seen during the clinical course. Surface antigen modulation and morphological changes, which were observed by microscopic examination and flow cytometry, were induced by in vitro incubation. Surface antigen modulation was more induced in the presence of phytohemagglutinine-conditioned media (PHA-CM) but the effects of PHA-CM on morphological changes were not clear.