In a clinical phase II trial, radiotherapy and escalating doses of paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) were given concurrently to patients with stage IIIA/B non-small cell lung cancer. Radiotherapy was given in daily doses of 2 Gy, 5 days a week, in weeks 1 to 3 and 6 to 8 for a total dose of 56 Gy. Paclitaxel was given in 3-hour infusions on day 1, also in weeks 1 to 3 and 6 to 8. The starting dose level was 50 mg/m2; the subsequent dose levels were 60, 72, 86, and 103 mg/m2. Cohorts of three to six patients were assigned to each dose level until intolerable toxicity (eg, World Health Organization grade 3 or 4 leukopenia) occurred in three of the six patients. Currently, 15 patients have entered the study. Twelve patients have finished the treatment protocol and are evaluable for toxicity and response. Hematologic toxicity was mild, and even at 86 mg/m2, the highest evaluable dose level, no severe myelosuppression was noticed. The main clinical toxicity was the occurrence of pulmonary infections, which were seen in five patients. One of these patients had a Pneumocystis carinii infection, but no pathogens were isolated from the four others with interstitial infections. Mild to moderate esophagitis was seen in five patients. All patients showed a decrease of tumor size. Four patients had a major response with nearly complete disappearance of radiologic tumor signs, five patients had a partial remission, and three patients experienced a minor response. Thus, the overall response rate was 75%. In summary, the maximum tolerated dose of paclitaxel has not yet been achieved, the occurrence of pulmonary infections represents the major clinical toxicity, and the extremely high response rate merits further clinical evaluation of this regimen.