A phase I study to determine the maximum tolerated dose and toxicity of gemcitabine when given as a 24 h infusion to patients with inoperable non-small-cell lung cancer (NSCLC). A total of 24 patients with unresectable stage IIIa-IV NSCLC were entered into the study. Gemcitabine was administered as a 24 h infusion on days 0, 7 and 14. Courses of therapy were repeated every 28 days. There were 16 males and 8 females with a median age of 51 years (range 40-73 years). The WHO performance score was 1 (21 patients) or 2 (3 patients). The TNM stage was IIIa (6), IIIb (10) and IV (8). Three patients were entered at each dose level with six at the maximum tolerated dose (MTD). Dose levels were 10, 20, 40, 80, 120, 180 and 210 mg m-2. The MTD was 180 mg m-2 and dose-limiting toxicity was neutropenia and lethargy. Partial response was observed in five (21%) patients (95% CI 7-42%) lasting 10, 14, 18, 47 and 51 + weeks. The maximum tolerated dose of gemcitabine given as a 24 h infusion was 180 mg m-2.