This study was designed to evaluate the therapeutic efficacy and toxicity of recombinant interferon alpha-2a (rIFN alfa-2a) given as initial systemic therapy in untreated mycosis fungoides and/or Sezary's syndrome patients, at a slowly escalating schedule up to the maximal tolerated dose. At the same time this schedule was administered in patients who had relapsed or were refractory to previous treatment; 28 newly diagnosed and 15 previously treated patients entered the study. IFN was given daily with dose escalation from 3 to 18 MU. The last follow-up in June 1990 indicates that 90% of previously untreated patients who obtained a complete remission remain in continuous complete remission after 18 to 40 months and that 75% of previously untreated patients who obtained partial remission remain in partial remission after 20-44 months. The event-free survival projected, calculated using the Kaplan and Meier product limit technique, was 21% of all patients at 54.7 months (40% in the previously untreated groups and 14% in the previously treated group: P = 0.12). In conclusion, interferon is very effective as a single agent in cutaneous T-cell lymphomas.