We performed a 3-armed phase III study between 1982 and 1990 to evaluate low dose natural interferon alfa (nIFN-alpha) as a maintenance therapy in small cell lung cancer (SCLC) following induction chemotherapy (CT) and consolidation radiotherapy (RT). All patients received four cycles of CT (cyclophosphamide, vincristine, etoposide), followed by split-course RT (55 Gy in 20 fractions over 7 weeks). 410 patients entered the study. 237 patients who completed induction CT + RT and were classified as responders (complete response + partial response) were randomly assigned to arm 1: low dose nIFN-alpha (91 patients); arm 2: maintenance CT, six cycles of CAP (cyclophosphamide, doxorubicin, cisplatin) (59 patients); or arm 3: control arm (no maintenance treatment) (87 patients). Halfway through the study the CAP arm was discontinued. There was no difference in median survival between the groups (IFN: 11 months, CAP: 11 months, control: 10 months), but a clear difference in long-term survival and in survival in the limited disease group, favouring nIFN-alpha maintenance therapy. Proportional hazards regression analysis also showed a significant effect of IFN treatment on survival. Our results suggest a role for nIFN-alpha in maintaining a clinically disease-free status achieved with other treatment modalities.