Concurrent twice-weekly paclitaxel and thoracic radiation (XRT) for stage III non-small cell lung cancer were studied in a phase I trial. Radiation was delivered in fractions of 1.8 to 2.0 Gy/d to a total dose of 61 Gy. Paclitaxel, at a starting dose of 25 mg/m2/d, was administered intravenously over 1 hour before daily XRT on Mondays and Thursdays for 6 weeks for a total of 12 doses. The paclitaxel dose was escalated by 5 mg/m2/d for each cohort of patients to determine the maximum tolerated dose. The highest dose of paclitaxel reached was 40 mg/m2, which resulted in dose-limiting toxicities of esophagitis and local skin desquamation. For each dose group, the median total number of paclitaxel doses administered was 12 and the median total XRT dose delivered was 61 Gy. The overall response rate was 80%. The overall median survival was 20 months and the 3-year survival rate was 20%. We conclude that the maximum tolerated dose of paclitaxel is 35 mg/m2 given twice weekly for 6 weeks concurrently with XRT. This study provides the basis for using paclitaxel, given twice weekly at 30 mg/m2, with weekly carboplatin and concurrent XRT for stage III non-small cell lung cancer in an ongoing phase II trial.