OBJECTIVE Both cisplatin (CDDP) and paclitaxel have shown good antitumor activity in non-small-cell lung cancer (NSCLC) patients and are able to potentiate the antitumor effects of radiation therapy (RT). This study aimed to determine the maximum-tolerated doses (MTDs) of CDDP and paclitaxel (escalated alternately) when given concurrently with RT and to define the nature of the dose-limiting toxicity (DLT). METHODS Chemotherapy-naive patients with locally advanced NSCLC received six weekly administrations of a CDDP-paclitaxel combination with concurrent local RT. The starting doses of CDDP and paclitaxel were 30 mg/m2/wk and 35 mg/m2/wk, respectively. RT was initially given at the dose of 1.2 Gy twice daily for 5 days per week for 5 weeks (total dose, 60 Gy) and at a single daily dose of 2 Gy for 5 days per week for 6 weeks in the last two cohorts of patients. The drug doses were escalated alternately until DLT occurred in more than one third of the patients in a given cohort. RESULTS Overall, 25 patients were recruited through five different cohorts. All were assessable for toxicity. Esophagitis was the main toxicity and occurred in 16 of 25 patients (64%) and was grade 3 or 4 in five of them. At step 3 (CDDP 35 mg/m2/wk and paclitaxel 45 mg/m2/wk), two of five patients had to discontinue treatment because of severe esophagitis and one of these died of complications related to grade 4 esophagitis. However, keeping the same doses of chemotherapy and replacing hyperfractionation with a standard single-day fraction, weekly doses of CDDP and paclitaxel of 35 mg/m2 and 45 mg/m2 could be safely administered. Neutropenia was by far the most relevant hematologic toxicity and occurred in 33 of 141 weekly delivered courses, but it was of grade 4 in only four courses. Substantial pulmonary or neurologic toxicity was not observed in this study. Two complete responses (CRs) and 13 partial responses (PRs) were observed, for a 60% overall response rate (95% confidence interval [CI], 39% to 79%). The median survival time was 16 months, with a 66% 1-year survival probability. CONCLUSIONS CDDP 35 mg/m2/wk and paclitaxel 45 mg/m2/wk can be safely administered with concurrent standard RT. The use of hyperfractionation is associated with a more frequent occurrence of severe esophagitis and requires a reduction of the CDDP dose to 30 mg/m2/ wk. Only future randomized trials will elucidate which of these two approaches (standard or hyperfractionated RT) is the better option to improve the outcome of patients with locally advanced NSCLC.