Both paclitaxel and ifosfamide have significant single-agent activity in non-small cell lung cancer. We designed a phase I study combining escalating doses of paclitaxel, administered by 24-hour infusion, with ifosfamide given at a dose of 1.6 g/m2 daily x 3. Paclitaxel dose levels were 135, 170, 200, 250, and 300 mg/m2. The goal of the study was to determine the maximum tolerated dose and dose-limiting toxicities of paclitaxel when used in this combination. Dose escalation was possible because of the use of filgrastim, a granulocyte colony-stimulating factor. Twenty-five patients at three institutions were treated. The dose-limiting toxicity of the combination was granulocytopenia, with other toxicities being generally mild to moderate. The maximum tolerated dose of paclitaxel was 300 mg/m2, and the recommended phase II dose is 250 mg/m2. There was a suggestion of a dose-response curve with paclitaxel as all three partial responses were seen at the 250 mg/m2 dose level. An additional II patients had objective regression or stable disease lasting for 9 to 30 weeks. A phase II study of this combination is currently being planned by the Cancer and Leukemia Group B.