In 100 patients with nonblastic Ph1-positive chronic granulocytic leukemia (CGL) the main features from the initial bone marrow biopsy were correlated with other clinical and hematological parameters and analyzed for prognostic significance. The mean number ( ± SD) of megakaryocytes per square millimeter of marrow tissue was 25.9 ± 18.8. Marked reticulin fibrosis was observed in 19% of the patients but in only 5 cases it was diffuse, whereas collagen deposition was found in 10. patients. Reticulin fibrosis correlated significantly with white blood cell count and peripheral blood blast cell percentage, but not with platelet count. Actuarial survival studies demonstrated that marked diffuse reticulin fibrosis (p = 0.0005) and more than 15 megakaryocytes/mm3 of marrow tissue (p = 0.01) were associated with a poorer prognosis. However, they lost their prognostic influence when included in a multivariate regression model together with the patients’ relative risk according to the currently most widely accepted CGL prognostic system. Such results indicate that bone marrow biopsy features add little prognostic weight to known prognostic indicators in CGL.