In a clinical phase II trial, escalating doses of paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) were given with concurrent radiation 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 through 3 and 6 through 8. Paclitaxel was given on day 1 of weeks 1 through 3 and 6 through 8, at a starting dose level of 50 mg/m2. Subsequent paclitaxel dose levels were 60, 72, 86, and 103 mg/m2. Three to six patients were included at each dose level until intolerable toxicity (World Health Organization grade 3 or 4 leukopenia) occurred in three of six patients. To date, 27 patients have entered the protocol. Hematologic toxicity was mild with no severe myelosuppression up to the 86-mg/m2 dose level. At paclitaxel 103 mg/m2, four of six patients developed grade 3 or 4 leukopenia, and dose escalation was stopped. The maximum tolerated dose was thus determined to be 86 mg/m2. The main clinical toxicity was the occurrence of pulmonary infections (seven patients), one of whom had Pneumocystis carinii infection; the six others had interstitial infections with no pathogen isolated. Mild to moderate esophagitis was seen in five patients. Thus far, of 24 patients evaluable for response, 18 showed decreased tumor size. Four patients achieved major responses (near-complete disappearance of radiologic tumor signs), 11 patients achieved partial remission, and three patients had a minor response. The overall response rate was 75%. In summary, the maximum tolerated dose of paclitaxel in this study has been determined to be 86 mg/m2 weekly. Pulmonary infections represent the major clinical toxicity, and the high response rate merits further clinical evaluation of this regimen.