Cisplatin-based chemoradiotherapy is becoming a standard treatment for patients with stage III non-small cell lung cancer (NSCLC). However, a significant proportion of patients with lung cancer also present with co-morbid conditions that indicate a poor prognosis and poor tolerance of treatment. We have completed a phase I/II study to evaluate the tolerability and efficacy of carboplatin-based chemoradiotherapy for patients with poor-risk stage III NSCLC. Twenty-four patients with stage IIIA/B NSCLC and concurrent medical conditions rendering them ineligible for cisplatin-based chemoradiotherapy protocols were treated with thoracic irradiation, 1.8 to 2 Gy daily to the primary tumor and regional lymph nodes, for a total dose of 61 Gy. Concurrently, patients received carboplatin 200 mg/m2/d intravenously on days 1, 3, 29, and 31, and etoposide 50 mg/m2/d intravenously on days 1 through 4 and 29 through 32. Among 23 assessable patients, 96% completed the two planned courses of chemotherapy and 87% completed the planned chest irradiation. Grade 3/4 toxicities included neutropenia in nine patients (39%), thrombocytopenia in five (22%), esophagitis in seven (30%), and nausea in two (9%). Four patients (17%) achieved a complete response and 16 (70%) a partial response, yielding an overall response rate of 87%. The median survival was 12 months, and the 2- and 3-year survival rates were 30% and 20%, respectively. In conclusion, this treatment regimen was relatively well tolerated, with promising response and survival in patients with poor-risk stage III NSCLC. This pilot study provides a basis for further investigation of this treatment regimen.