Background: To determine a recommended dose (RD) of chemoradiotherapy with use of involved-field conformal radiotherapy and accelerated hyperfractionation (AHF) for stage III non-small cell lung cancer (NSCLC). Methods: Eligible patients had unresectable stage III NSCLC, age of less than 75 years, PS: 0 or 1, V20 of 35% or less. PET was used for staging. Cisplatin (80mg/m²) was administered on day 1 and vinorelbine (20mg/m²) was administered on days 1 and 8 for two cycle. Twice-daily radiation therapy (1.5 Gy per fraction) without elective nodal irradiation started on day 1. Total doses were 60Gy in 40 fractions and 66Gy in 44 fractions at levels 1 and 2 respectively. After concurrent chemoradiotherapy, consolidation chemotherapy regimen was cisplatin (80mg/m²) on day 1 and vinorelbine (20mg/m²) on days 1 and 8 every 4 week for three cycles. The dose-limiting toxicity (DLT) was defined as grade ≥ 3 esophagitis, grade 3 neutropenic fever, grade ≥ 3 other non-hematologic toxicities and interruption of irradiation for more than 2 weeks. DLT was monitored for 90 days. Results: A total of 12 patients were enrolled (6 patients in Level 1, 6 patients in Levels 2). DLTs were noted in 2 patients at Level 1, which were grade 3 esophagitis and grade 3 febrile neutropenia. Radiation dose was escalated up to 66 Gy in 44 fractions (Level 2), and there was no DLT. In principle, Sixty-six Gy in 44 fractions (Level 2) should be the RD. Major toxicities were leucopenia, neutropenia, and anemia. The response rate, the median progression free survival time, and the median overall survival time was 83.3%, 10.4 months, and 36.3 months for all patients, respectively. Conclusions: The RD was 66 Gy in 44 fractions (Level 2). The toxicity of this chemoradiothrapy regimen was manageable and efficacy is promising. The efficacy and safety of this regimen should be confirmed in a phase II study. Clinical trial information: UMIN000003769.