Preclinical data from studies of human lung cancer xenografts suggest that the cytotoxic effects of cisplatin are enhanced by alpha-interferon. To verify the above observations, the authors initiated a Phase II trial in advanced non-small cell lung cancer (NSCLC). Cisplatin was given at 100 mg/m2 during a 28-day cycle in a divided day 1 and day 8 schedule. Starting on day 1, alpha-2B interferon was administered intramuscularly at a dose of 5 million units three times a week continuously for a minimum of 2 months. Between January 1989 and September 1989, 30 patients were evaluated for response and toxicity. According to the staging system proposed by Mountain, 20 patients had Stage IV disease, 7 had Stage IIIB disease, and 3 had Stage IIIA disease. Expression of neuron-specific enolase (NSE) and Leu-7 was immunohistochemically investigated to evaluate possible relationship to treatment response. The response rate was 13.3% (95% confidence interval [CI]: 1.2% to 25%). The four responders showed positivity for NSE, and two of them were positive for Leu-7. An average of three cycles was given. The mean dose intensity administered was 83% of the projected dose for cisplatin and 92% of the projected dose for alpha-2B interferon. A standard scale was used to assess interferon toxicity. Hematologic, renal, and systemic side effects were not significant. In advanced NSCLC the addition of alpha-2B interferon did not increase the cisplatin-induced response rate. Further studies should be performed to determine the real value of chemotherapy response in tumors showing positive immunoreactivity for neural markers such as NSE and Leu-7.