Three cases of chronic myelogenous leukemia (CML) were studied, occurring in 22, 43, and 30-year-old-men. Two observations (nos. 1 and 3) concerned typical CML, treated by discontinuous busulfan; in the last patient (no. 2), also presenting in addition with a constitutional deficiency from Hageman factor, diagnosis (Ph 1 chromosome) was based on a moderate leukocytosis with myelemia, spontaneously regressive for more than one year. Chronic phase duration was 17, 16 and 8 months respectively. During the first blast crisis, abnormal cells were rather of granular type in one case (no 1), undifferentiated in the other two. In the three observations, complete remission was easily obtained with prednisolone - vincristine but revealed very brief; 2, 2 and 4 months. Among the three patients a second blast crisis was preceded, in two cases (nos. 1 et 3), by a new CML phase during 3 and 1 months respectively. Treatment was then purely palliative by 6-mercaptopurine and hydroxyurea.