This trial was conducted to determine the maximum-tolerated dose (MTD), principal toxicity, and recommend dose for phase II study of the combination of gemcitabine and nedaplatin in patients with advanced non-small cell lung cancer (NSCLC). Patients with previously untreated NSCLC were eligible if they had a performance status of 0-2, were 75 years or younger, and had adequate organ function. The doses of gemcitabine (days 1, 8) and nedaplatin (day 1) studied were 800/60, 800/70, 800/80, 1000/80, and 1000/100 (mg/m(2)), repeated every 3 weeks. Toxicity could be assessed in all 21 patients enrolled, response could be assessed in 20 patients. The patients were 12 men and 9 women with a mean age of 69 years (range, 47-75 years). Four patients had stage IIIB disease and 17 patients had stage IV disease. The most common histologic type was adenocarcinoma. The MTD was not reached even at the highest doses. The most frequent toxic effects were thrombocytopenia and neutropenia: grade 3 or 4 thrombocytopenia was observed in 19% of patients, and grade 3 or 4 neutropenia in 24% of patients. Nonhematologic toxicities were mild. Grade 3 hepatic dysfunction occurred in 3 patients. Relatively few patients required dose modifications. The median dose-intensities were 91.5 and 93.1%, respectively, of the planned doses of gemcitabine and nedaplatin. The overall response rate was 35% (95% confidence interval, 15.4-59.2%). All responses were seen above level 3. The MTD was not reached even at the highest combination doses. We recommend doses of 1000 mg/m(2) of gemcitabine and 100 mg/m(2) of nedaplatin for phase II study. This combination chemotherapy is active and well tolerated and warrants phase II study.