Sixteen patients in chronic myeloid leukemia blast crisis entered a phase II trial with vindesine and prednisone. Median duration of the chronic phase was 29 months in 13 patients, one previously had polycythemia vera, and two presented with a primary blast crisis. Eleven patients had myeloblastic features, as evidenced by morphology, cytochemistry, and cell surface antigens; three had a mixture of myeloid and lymphoid blast cells with lymphoblastic predominance; one had blast cells which displayed lymphoid characteristics; and one was classified as undifferentiated. Three patients had complete remissions lasting 1 month (myeloid), 3 months (mixed), and 5 months (lymphoid). Eleven patients had minor responses, with a median duration of 3 weeks (eight with myeloid, two with mixed, and one with undifferentiated). Two patients did not respond to vindesine. Leukopenia and thrombocytopenia were severe and prolonged independent of their morphologic or immunologic phenotype.