Treatment of Philadelphia (Ph) chromosome-positive chronic myelogenous leukemia (CML) with recombinant interferon-alpha (IFN-A) results in complete disappearance of the Ph chromosome in about 10% to 15% of patients in early chronic phase. This group has a long survival and very low incidence of blast crisis. The first known case is reported of extramedullary blastic transformation in a patient with medullary complete cytogenetic response (0% Ph-positive metaphases) to IFN-A. Four episodes of extramedullary blast crisis have occurred in this patient. The first three episodes were lymphoid by morphology and cytochemical stains. Molecular analysis confirmed breakpoint cluster region rearrangement. The most recent transformation was myeloid in nature and involved bone and pulmonary parenchyma. The patient is currently undergoing a second autologous transplantation with stored bone marrow that is Ph negative. The patient has survived more than 18 months since the first episode of blast crisis, and the bone marrow is normal.