The morphologic, cytogenetic, and clinical features of 58 patients with transformation of Philadelphia chromosome (Ph1) positive chronic myelogenous leukemia (CML) were evaluated. The patients were divided into two groups on the basis of blood and marrow findings: blast crisis and subacute transformation. The evolution of the leukemic process in 41 patients was classified as blast crisis based on one of three criteria: 30% or more blasts in blood and/or marrow smears, intramedullary focus of blast transformation in a marrow trephine biopsy, or blast transformation in an extramedullary site. The 17 patients with subacute transformation of CML had a deteriorating clinical and hematologic picture but did not manifest any of the criteria for blast crisis. The blood and marrow findings in this group of patients were characterized by several qualitative and quantitative changes, including anemia, thrombocytopenia, decreasing leukocyte count, increasing basophilia, myelofibrosis, dysplastic alterations in hematopoietic cells, and increased blasts which, however, never exceeded 25%. Chromosome abnormalities in addition to the Ph1 were found in 65% of the patients with blast crisis and 86% of the patients with subacute transformation. The 41 patients with blast crisis had a median survival of nine weeks; the 17 with subacute transformation had a median survival of 26 weeks. The shortest median survival for patients with blast crisis, four weeks, occurred in the patients with myeloid blast crisis with chromosome abnormalities in addition to the Ph. The longest median survival, 52 plus weeks, occurred in patients with lymphoid blast crisis with only the Ph1 at transformation.