The prognostic value of clinicopathological parameters, recorded at diagnosis, in idiopathic myelofibrosis (IMF) was retrospectively analyzed in a consecutive series of 32 patients followed for a minimum of 5 and maximum of 134 months in the period 1983-1994. Of the 32 patients, 18 were males and 14 were females. The mean age was 59.8 years (S.D., 10.4; range, 44-78 years). At the time of closure of the study (May 1995), 10 patients were dead, 18 were alive, and four were lost to follow up. The mean survival was 41 months (range 2-130 months). Univariate analysis showed that the following features were associated with a significantly shorter survival: (1) anemia (hematocrit <30%), (2) platelet count < 100,000/mm3. Age, sex, size of spleen and liver, WBC count, increased reticulin and collagen fibrosis were not of prognostic significance. Clinicopathological correlation was found between the sinusal hemopoiesis and the number of WBC precursors in peripheral blood, and between WBC precursors and spleen size. Medullary fibrosis was found to be associated with striking predominance of large and very mature megakaryocytes. Erythroid hyperplasia in the bone marrow correlated negatively with WBC precursors and spleen and liver size, and positively with reticulocyte count.