Bone marrow trephine biopsies from 30 cases of prolymphocytic leukemia (PLL), 16B and 14T were studied. The bone marrow infiltration by the neoplastic prolymphocytes showed the following four histological patterns: interstitial, mixed (interstitial-nodular), mixed (interstitial-diffuse), and diffuse. The most frequent pattern was the mixed (interstitial-nodular) seen in 40% of cases. In comparison with available data from patients with chronic lymphocytic leukemia, the bone marrow involvement in PLL differs by the absence of the pure nodular pattern and by the presence of a mixed (interstitial-diffuse) type, seen in 5 cases. No significant differences between the histological patterns in B- and T-PLL were observed. All the cases reviewed had reticulin fibrosis which was more intense in the T cases and in those with a diffuse pattern. The findings described here will help in the further characterization of these lymphoproliferative disorders.