Background: The optimal dose of radiotherapy remains unclear in concurrent chemoradiotherapy for unresectable stage III NSCLC. We previously concluded that the recommended dose for further trial was 72Gy in a phase I study (Sekine et al., Int J Radiat Oncol Biol Phys. 953-959, 2012). Methods: Eligible patients (unresectable stage III NSCLC, age between 20 and 74, PS 0-1, V₂₀ ≤ 30%) received cisplatin (80 mg/m² day 1) and vinorelbine (20 mg/m² days 1 and 8) repeated every 4 weeks for 3-4 cycles. The 3D-CRT started at the first day of chemotherapy at a total dose of 72 Gy in 36 fractions. The primary endpoint was a 2-year survival rate and the planned sample size was 60 to reject the rate of 45% under the expectation of 65% with a power of 90% and an alpha error of 5%. Results: Thirty-one patients from 4 institutions were enrolled between May 2009 and March 2010. This trial was terminated early due to slow accrual and grade 5 pulmonary toxicities in 2 patients. There were 25 men and 6 women with a median (range) age of 59 (32-72) years. Of these, 23 had adenocarcinoma and 21 had stage IIIA disease. The median (range) V₂₀ value was 20 (9-30). The full planned dose of radiotherapy could be administered in 30 (97%) patients, and 26 (84%) of the patients received 3-4 cycles of chemotherapy. During the chemoradiotherapy, grade 3-4 febrile neutropenia, infection and esophagitis were noted in 5, 4 and 1 of the 31 patients, respectively. After completion of the planned chemoradiotherapy, 5 patients had grade 3 or higher radiation pneumonitis, and 2 (6%) of these patients died at 6.6 and 7.3 months after the treatment started. The overall response rate was 97% (95% confidence interval: 83.3-99.9). Twenty-four patients are alive and thirteen patients experienced recurrence (2 had loco-regional recurrences, 7 had distant recurrence and 4 had mixed recurrence pattern) at a median follow up of 16.4 months. Conclusions: Concurrent high-dose (72Gy) 3D-CRT with chemotherapy using cisplatin and vinorelbine may have a too excessive incidence of pulmonary toxicities to warrant any further evaluation.