The bone marrow of five patients with progressive hairy cell leukemia was examined histologically and by magnetic resonance imaging in a prospective study. Iliac crest biopsies and magnetic resonance scans were performed before and after nine months of therapy with pentostatin (four patients) and alpha-interferon (one patient). T1-weighted scans were evaluated quantitatively and in terms of their visual appearance in three regions of interest (lumbar spine, pelvis and femur). In contrast to bone marrow histology, it was possible to detect differences in the degree of infiltration between these marrow regions in four patients by magnetic resonance imaging. After treatment, three patients had no residual bone marrow infiltration as determined histologically; in parallel, the magnetic resonance images had normalized. The remaining two patients achieved partial remission: marrow infiltration was estimated to be 20% histologically, corresponding well to the signal reduction obtained by magnetic resonance imaging. These data suggest that magnetic resonance imaging of the bone marrow is a sensitive method for assessing responses to treatment with pentostatin and alpha-interferon in patients with hairy cell leukemia.