The prognostic value of different clinical and laboratory findings at diagnosis of chronic myeloid leukemia (CML) was analyzed in a series of 121 cytogenetically studied patients. From the univariate and multivariate analysis of the whole series it was apparent that the minority of Ph1-negative patients (11.5%) could be considered as a poor prognosis group. The analysis was then restricted to the Ph1-positive patients. From a multivariate survival analysis (Cox's regression model) of the latter group the following poor prognosis factors emerged: splenomegaly, hepatomegaly, presence of erythroid precursors in peripheral blood, and bone marrow myeloblasts over 5%. From the contribution of each one of these factors to the regression model, a clinical staging of Ph1-positive CML was derived: stage I (low risk, 32% of patients), including patients with one or no factors; stage II (intermediate risk, 38%), including cases with two factors; and stage III (high risk, 30%), including patients with three or four factors. The difference in survival of the patients at different stages was highly significant (p less than 0.001).