Clinical-morphological study of myelofibrosis was carried out in patients with chronic lymphocytic leukemia at the debut of the disease. Trephinobiopsy specimens of the ileac bone, aspirated specimens of the bone marrow, and peripheral blood smears were studied in 80 patients. Chronic lymphocytic leukemia was associated with myelofibrosis of different severity in 22.5% cases. Morphometric analysis of trephinobiopsy specimens showed that the severity (histology and dissemination) of myelofibrosis correlated with the type of tumor involvement of the bone marrow. Focal tumor involvement of the bone marrow predominated in trephinobiopsy specimens from patients without myelofibrosis, while patients with myelofibrosis developed mainly diffuse tumor infiltration, associated with the greatest dissemination of the initial and manifest myelofibrosis. No myelofibrosis was found in patients with interstitial tumor involvement of the bone marrow. The severity of the initial and manifest myelofibrosis directly correlated with the tumor involvement of the bone marrow and peripheral blood. Evaluation of the prognosis showed that initial myelofibrosis was associated with disease standing of 5.5 months, while manifest condition with a disease of 8.5 months and longer.