Sixteen patients affected by previously untreated non-small cell lung cancer stage IIIB or IV received radiotherapy and paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) as radiation sensitizer in an open, nonrandomized pilot study to find the maximum tolerated dose of the drug concomitantly combined with radiation. Paclitaxel was given as a 3-hour infusion once weekly at a dose escalating by 10 mg/m2/wk for every patient cohort, starting at 40 mg/m2/wk and continuing to 80 mg/m2/wk. Conventionally fractionated (2 Gy/d for 5 d/wk for 5 weeks) radiotherapy up to 50 Gy was delivered to the primary tumor and mediastinum with a 6-mv linear accelerator. Hematologic toxicity has been very low; grade 3 World Health Organization nonhematalogic toxicities have been registered only at the 80 mg/m2/wk dose level. Seven patients achieved a major response, three patients had stable disease, and five patients progressed; three patients are still responding, whereas the others are relapsed and five of them died of disease. The median duration of response was 5 months. Paclitaxel may be safely combined with radiation at the maximum tolerated dose of 70 mg/m2/wk. Our data seem to confirm the radiosensitizing effect of the drug, independent of the dose level. Low doses of paclitaxel given as a single agent are unable to control metastatic disease.