Thirty hairy cell leukemia patients were evaluated repeatedly for their bone marrow (BM) histology. At the time of diagnosis, 18 (60%) had diffuse, 9 (30%) had interstitial, and 2 (10%) had a mixed (diffuse and interstitial) pattern of BM disease. The follow-up BM specimens were obtained at intervals of 3-24 months, and the follow-up observation period was 12-94 months. In patients who were nontreated or only splenectomized, no significant changes were observed except of a persistent megaloblastoid picture of the red cell series and an increase of BM fibrosis. In the alpha-interferon treated patients a complete disappearance of hairy cells was observed in one and a dramatic reduction in five. The hairy cell index was reduced from a mean of 0.8 before to 0.1 after alpha-interferon therapy; most patients displayed megaloblastoid erythropoiesis. In the complete responder features of myelodysplasia were present.