Background: Published studies on clinical outcome of helical tomotherapy for lung cancer are limited. The purpose of this study was to evaluate clinical outcomes and treatment-related toxicity in inoperable non-small cell lung cancer (NSCLC) patients treated with helical tomotherapy in Korea. Materials and Methods: Twenty-seven patients with NSCLC were included in this retrospective study. Radiotherapy was performed using helical tomotherapy with a daily dose of 2.1-3 Gy delivered at 5 fractions per week resulting in a total dose of 62.5-69.3 Gy. We assessed radiation-related lung and esophageal toxicity, and analyzed overall survival, locoregional recurrence-free survival, distant metastasis-free survival, and prognostic factors for overall survival. Results: The median follow-up period was 28.9 months (range, 10.1-69.4). The median overall survival time was 28.9 months, and 1-, 2-, and 3-year overall survival rates were 96.2%, 92.0%, and 60.0%. The median locoregional recurrence-free survival time was 24.3 months, and 1-, 2-, and 3-year locoregional recurrence-free survival rates were 85.2%, 64.5%, and 50.3%. The median distant metastasis-free survival time was 26.7 months, and 1-, 2-, and 3-year distant metastasis-free survival rates were 92.3%, 83.9%, and 65.3%, respectively. Gross tumor volume was the most significant prognostic factor for overall survival. No grade 4 or more toxicity was observed. Conclusions: Helical tomotherapy in patients with inoperable NSCLC resulted in high survival rates with an acceptable level of toxicity, suggesting it is an effective treatment option in patients with medically inoperable NSCLC.