Gemcitabine is a novel nucleoside analog with unique activity against a wide range of solid tumors. We initiated a multicenter phase II study in patients with non-small cell lung cancer (NSCLC) to evaluate the efficacy and safety of gemcitabine. Eligible patients had stage III and IV, previously untreated with chemotherapy, age range from 18 to 80 years, and ECOG performance status 0 2. Gemcitabine was administered at 1000 mg/m2 as a continuous i.v. infusion once a week for a consecutive 3 week period, followed by 1 week of rest. Of the 69 patients enrolled, 67 patients were eligible for efficacy evaluation. The overall response rate was 20.9% with a 95% confidence interval of 11.9-32.6%. The median survival time was 9.0 months and the 12 month survival rate was 31.3%. Grade 3 or 4 toxicities included neutropenia in 22.7%, anemia in 13.4%, leukopenia in 10.4%, anorexia in 10.4%, malaise in 7.5% and nausea/vomiting in 6.0%. Serious toxicities were septic shock and interstitial pneumonia (one patient each). Gemcitabine, administered weekly for three consecutive weeks followed by 1 week of rest, is an active agent for NSCLC. Gemcitabine is currently being evaluated in combination with cisplatin and other agents.