Both recombinant interferon alpha and deoxycoformycin (dCF) are effective in the treatment of hairy cell leukaemia. In an attempt to reduce the complications from dCF therapy, a pilot study of the Eastern Cooperative Oncology Group (ECOG) first treated patients with interferon to improve peripheral blood cell counts before dCF treatment began. Thirty-four patients were treated for 3 months with recombinant interferon alpha-2a (rIFN alpha-2a), 3 x 10(6) IU subcutaneously three times a week for 3 months, and then by dCF, 4 mg/m2 intravenously every 2 weeks for a maximum of 12 months. The overall response rate was 94% (32/34); 76% of patients (26/34) had complete response (CR) (90% confidence interval, 62-88%) and 18% (6/34) partial response. One patient was found to have a Mycobacterium avium infection while receiving rIFN alpha-2a. Without specific antimycobacterial therapy and with continued administration of rIFN alpha-2a and dCF, the infection resolved and he achieved CR. Three patients had culture-negative febrile episodes during the dCF phase of treatment. Non-disseminated herpes zoster developed in four patients, but three of the episodes occurred only after treatment was discontinued. Sequential administration of rIFN alpha-2a and dCF resulted in fewer infections (P = 0.027) than in ECOG's previous study of dCF used alone. Two patients died, one of combined hairy cell leukaemia and non-Hodgkin's lymphoma of intermediate histologic type 17 months after entry into the study and the other of cardiac arrest 20 months after entry. Thirty-two patients were alive with a median follow-up of 21 months (range 13-31 months). This combination produces durable CRs with a low incidence of infection.