Escalating doses of recombinant interferon alfa-2a (Roferon-A), administered intramuscularly three times weekly, combined with psoralen plus ultraviolet light irradiation (PUVA), were tested in a phase I trial for the therapy of patients with cutaneous T-cell lymphomas (CTCL). Interferon doses were escalated in groups of three patients from 6 million to 30 million IUs three times weekly. Disease stages ranged from IB to IVB. Eighty percent of the patients entered in this trial had failed at least one prior therapy. Complete remissions were obtained in 12 of 15 patients, and partial responses were seen in two of 15 patients, for an overall response rate of 93%. The median duration of response exceeded 13 months (range, 3-15+). All patients who responded have been maintained on therapy. The dose-limiting toxic effects were constitutional symptoms such as fevers and malaise (93.3%), leukopenias (40.0%), mental status changes consisting of depression and confusion (33.3%), and photosensitivity (26.6%). These side effects were reversible with a decrement in dose or discontinuation of the interferon. No patient tolerated 30 million IU of the interferon for extended periods; the maximally tolerated dose was 18 million IU. Interferon plus PUVA appears to be a highly effective regimen for the treatment of patients with CTCL. Phase II studies investigating this combination, using 18 million IU of interferon alfa-2a three times weekly, should be undertaken to expand these findings, and to attempt to reduce the toxic effects associated with this therapy.