2-Chlorodeoxyadenosine (2-CdA) yields high complete remission (CR) rates in patients with hairy cell leukemia (HCL). In an effort to detect minimal residual disease, we studied two B-lineage antibodies, L26 and MB2, and a T-lineage antibody, UCHL-1, in fixed marrow core biopsies from 34 patients with HCL before and after 2-CdA. Before therapy, hairy cells exhibited intense cytoplasmic membrane reactivity with L26 and strong intracytoplasmic reactivity with MB2. UCHL-1 did not react with hairy cells. Thirty-one patients were assessable 3 months after therapy. Five of 24 (21%) patients in CR by routine evaluation had residual HCL detected by immunostaining. Four of these 5 patients have been reevaluated at 1 year. One patient relapsed by routine evaluation, 2 remained positive by immunostaining alone, and 1 patient became negative by immunostaining. A total of 19 patients have been evaluated at 1 year. Only 1 additional patient has become positive by immunostaining alone. Immunostaining using the B-lineage antibodies highlighted the presence of hairy cells with preservation of morphology. This assisted in quantifying the extent of disease, particularly when hairy cells were interstitial and blended with surrounding hematopoietic tissue, when hairy cells were present in hypocellular marrows, when hairy cells were spindle-shaped, and when marrows were markedly fibrotic. Because immunostaining can be easily performed on routinely processed marrows, it is an attractive method to detect minimal residual disease. Our data suggest that some patients in apparent CR after 2-CdA may have minimal residual disease. Patients will need to be observed prospectively to determine if residual disease will be predictive of relapse.