Origin and spread of the chromosomally abnormal cells that appear in chronic myeloid leukemia (CML) after transformation are unknown. Spleen and lymph node may be involved. In 16 patients with CML splenectomy and/or adenectomy were performed before or during the blastic crisis of the disease, followed by a chromosomal analysis of the cells from the removed organ. At the same time, the chromosomes of the blood cell and of the bone marrow were also analyzed. Analyses were done with R banding. The results show that an extramedullary clonal development with duplication of the Ph1 chromosome and other features occurred. From a cytogenetic standpoint, acute blastic phase of CML is frequently characterized by an increased number of chromosomes owing to preferential gain of additional chromosomes. This, then, would clearly point to extamedullary acute transition in CML.