Background: S-1 is an oral anticancer agent containing a mixture of tegafur, 5-chloro-2, 4-dihydroxypyridine, and potassium oxonate, which has been shown to have a radiosensitizing effect in preclinical models. Recent phase II studies have shown that chemoradiotherapy using cisplatin and S-1 yields promising PFS durations of 15 to 20 months in non-elderly (less than 75 years old) patients with locally advanced NSCLC. We conducted a phase II study of CBDCA, S-1 and TRT for elderly patients with locally advanced NSCLC. Methods: The eligibility criteria included patients with unresectable stage III NSCLC, a chemotherapy-naïve status, PS 0 to 1, and age ≥ 71 years. Patients received CBDCA (AUC 3) on day 1 and S-1 (40mg/m² twice daily) on days 1 to 14, q4w, up to 4 cycles, plus concurrent TRT at a total dose of 60Gy. The primary endpoint was 1-year PFS rate. The sample size was set at 28 patients, with a one-sided alpha of 0.05, beta of 0.2, and expected and threshold values for primary endpoints of 50% and 30%. Results: Between Feb 2014 and Dec 2016, 28 patients were enrolled in this study. All 28 patients were eligible and assessable. Baseline characteristics as follows: median age (range) 77 (71-83) years; women, n = 3 (11%); ECOG PS of 0, n = 15 (54%); stage IIIB, n = 8 (29%); tumor histopathological type: adeno, n = 12 (43%), squamous, n = 13 (46%), and NSCLC-NOS, n = 3 (11%). All 4 cycles of CBDCA plus S-1 could be completed in 15 patients (54%). TRT at 60 Gy was completed in 26 patients (93%). Response rate was 71%. Grade 3-4 toxicities included neutropenia (7%), thrombocytopenia (21%), hypoalbuminemia (7%), hyponatremia (11%), anorexia (7%), fatigue (7%), and colitis (7%). Grade 3 radiation pneumonitis was observed in 5 patients (18%). No treatment-related death was observed. The 1-year PFS rate was 56% (90% CI, 40.0 to 71.7%), and the median PFS was 13.9 months (95%CI, 4.8 to 23.0 months) with a median follow-up period of 18.2 months in censored cases. Conclusions: Combination chemotherapy of CBDCA plus S-1 and concurrent TRT had promising efficacy in elderly patients with locally advanced NSCLC; however, radiation pneumonitis was frequently observed. Clinical trial information: UMIN000005794.