Of 349 consecutive patients with Philadelphia-positive chronic myelogenous leukemia (Ph' + CML), 14 (4%) developed extramedullary disease (EMD) during their illness. The sites of EMD were: bone (57%), lymph nodes (29%), skin and soft tissues (21%), central nervous system (14%). The median time from diagnosis of CML to the occurrence of EMD was 48 months. At the time of diagnosis of EMD, 7 patients were hematologically in chronic phase, while 7 showed features of accelerated or blastic CML. For patients lacking medullary blastic transformation criteria, the median time from diagnosis of EMD to blast crisis was 4 months. The overall median survival from development of EMD was 5 months. In conclusion, EMD may occur during the course of CML either in the context of a frank blastic transformation, or as an isolated tumoral infiltrate which heralds an impending blast crisis. Its recognition requires a prompt change to acute-phase chemotherapy.