14 patients with HCL were started on alpha-interferon (IFN) treatment. Bone marrow changes were followed in trephine biopsies every 3rd month. Differential counting was performed according to the point counting method and fiber content was evaluated. At the start of treatment all patients had 80% or more of tumor cells in the bone marrow. The reduction of granulopoiesis was more pronounced than for erythropoiesis. During treatment cellularity decreased in 6/13 patients already after 3 months; later, cellularity was normalized in most patients. Only 1 patient showed a complete remission. 12/14 patients showed a significant but slow reduction of tumor cells. Also, between 12 and 18 months an improvement was seen in some patients. The megakaryocytes improved rapidly and the erythropoiesis was normalized before the granulopoiesis. Only 1 patient did not show a bone marrow response. 5/14 patients showed a reduction of bone marrow fibers, first seen after 6 months of treatment. IFN was discontinued in 9/14 patients. 7 of these relapsed during the observation period (18 months).