We studied changes in peripheral blood and bone marrow biopsy specimens obtained before, during, and after recombinant alpha 2b-interferon (IFN-alpha 2b) therapy in 25 patients with hairy cell leukemia. During therapy, only 1 patient showed no improvement in at least one of the parameters monitored. Granulocytopenia, thrombocytopenia, and monocytopenia resolved in 19/20, 14/15, and 17/18 patients, respectively. In 18/21 patients with Hb less than 12g/dl before treatment, the anemia became less severe. Hairy cells disappeared or decreased in numbers in the peripheral blood in all patients. In the bone marrow, numbers of hairy cells decreased and numbers of granulocytic, erythroid, and megakaryocytic cells increased usually within 3-6 months after the start of therapy. In no patient were hairy cells ever completely absent from the bone marrow. After cessation of IFN-alpha, the median Hb value, WBC, and platelet counts changed little for up to 12 months, but the absolute neutrophil count and absolute monocyte count decreased. Hairy cells reappeared in the peripheral blood of three patients. In the bone marrow the percentage of hairy cells increased, whereas the percentage of granulocytic and erythroid cells decreased. Neutrophil alkaline phosphatase (NAP) scores were abnormally high in 18/18 patients studied prior to IFN-alpha, but became normal in 17 of these during therapy and were normal in seven first studied during therapy. The median NAP score doubled by 3 months after cessation of therapy and was abnormal in 17/19 patients followed for 6 months. NAP score may be useful in predicting changes in the bone marrow in patients treated with IFN-alpha. We did not find any parameter in the pretherapy specimens that would have allowed us to predict individual response.