[en] Purpose: To evaluate the role of definitive radiation therapy (RT) in the treatment for medically inoperable patients with stage I-II non-small cell lung cancer (NSCLC). Materials and Methods: From 1976 through 1989, 84 patients with clinical stage I and II NSCLC were treated with definitive RT alone at Gunma University hospital. All patients were treated with 10 MV X-rays using antero-posterior parallel opposed fields. The total dose ranged from 60 Gy to 90 Gy (35 pts; 60-69 Gy, 39 pts; 70-74 Gy, 10 pts; ≥ 80 Gy) with once-daily standard fractionation. Results: The two and five-year survival rates were 74% and 31% for 28 patients with stage I disease, as compared with 40% and 19% for 56 patients with stage II respectively (p<0.05). Although there was no significant difference of survival rates by the histologic subtypes, in the patients with squamous cell carcinoma there were more long-term survivors. Fifty-three patients with tumors less than 5 cm in diameter had an infield progression rate of 14% at two years, in comparison with 38% of 31 patients with tumors greater than 5 cm (p<0.05). Overall distant failure occurred in 57% of the patients with smaller tumors and in 80% of the patients with larger tumors (p<0.05). The difference of survival rates for these two groups was statistically significant (p<0.005). Ten patients given a total dose of 80Gy or over had only 17% local progression at the time of last follow-up, however they had not been alive beyond three years because they developed pulmonary insufficiency due to severe stenosis of the proximal bronchus. For age and sex, there were no significant differences in survival, however, patients with performance status of 0-1 lived longer than those with a status of 2 or more (MST 24 versus 13 months; p=0.06). Conclusion: The tumor size was the most important factor not only for local control but also for distant failure. It was also suggested that the optimal radiation dose for medically inoperable stage I-II NSCLC ranged from 60 Gy to 70 Gy with once-daily standard fractionation