Between 1959 and 1973 were analyzed the records of 798 patients with chronic myelocytic leukemia. Mean survival (MS) for the entire group is 42 months. 342 patients have been followed closely during and after development of blastic transformation. Presence of following symptoms at the time of diagnosis: asthenia, weight loss, bone pain, fever, sweats and digestive disorders is of poor prognosis significance (MS: 36 months, no sign: MS 75 months) (P less than 0.001). Spleen size is also a prognostic factor. MS are respectively 70, 52 and 35 months if initial splenomegaly is moderate (less than 3 cm), marked (less than 6 cm) or tumoral (greater than or equal to 6 cm). Thrombocytopenia (less than 15,000/mm3 or thrombocythemia (greater than 1 million/mm3) have a poor prognosis with median survival 22 months and 28 months. If peripheral blast cells (hemocytoblasts + myeloblasts) exceed 5%, the prognosis is worse; beyond 10% MS is 26 months. In contrast certain factors have better prognosis: hemoglobin greater than or equal to 14 g/100 ml, young age (less than 20 y.) MS: 62 months), female sex and an initial WBC count below 25 x 10(3)/mm3 (MS: 70 months).