We have investigated the use of 2′-deoxycoformycin (DCF), a potent inhibitor of adenosine deaminase (ADA), in the treatment of 4 patients with advanced mycosis fungoides (MF). Since DCF has demonstrated an adverse effect in vitro and in vivo on the survival of leukemia T-cell lines, it appeared reasonable to examine its effect in patients with advanced cutaneous T-cell lymphoma. A total of 8 courses of DCF were given to the 4 patients. Since this study was part of an ongoing phase I investigation, each patient received a fixed dose (varying from 4 mg/sq m to 10 mg/sq m daily for 3 consecutive days) on a 28-day schedule. One patient had reversible renal insufficiency. Three patients had reversible myelosuppression. Two patients had a complete remission of disease for 7+ and 9+ mo. respectively. Two additional patients had partial remissions for 4 and 9 mo, respectively. We concluded that effective antitumor activity in advanced MF can be achieved with DCF at doses that may not be associated with prohibitive toxicity. We would encourage further investigation of this agent in patients with advanced cutaneous T-cell lymphoma.