The clinical course of 196 patients with chronic myelocytic leukemia (CML) was studied. Prognostic factors were analyzed using a standard prognostic model. From a univariate analysis of patients with nonblastic Philadelphia chromosome-positive CML, splenomegaly, bone marrow fibrosis, percentage of blasts and promyelocytes in the peripheral blood and LDH activity were shown to be factors with a significant negative influence on survival. However, age and the platelet count did not influence survival. The standard prognostic model, generated with the 4 variables (1) percentage of blasts and promyelocytes, (2) spleen size, (3) platelet count and (4) age did not provide a useful representation of risk status in this heterogenous patient population. However, the addition of further variables (LDH, additional chromosomal aberrations, percentage of basophiles/eosinophiles and percentage of bone marrow blasts) to the standard model allowed a separation into 2 patient groups: one with low and the other with intermediate to high risk. Our data support the general validity of the prognostic model; however, the applicability of the model may be compromised in hematologic centers with a heterogenous CML population due to the selection of high-risk patients. In this situation additional risk factors may have to be added to the prognostic formula.