Purpose; The phase III PROCLAIM study evaluated overall survival (OS) of concurrent pemetrexed-cisplatin and thoracic radiation therapy (TRT) followed by consolidation pemetrexed, versus etoposide-cisplatin and TRT followed by nonpemetrexed doublet consolidation therapy. Patients and Methods: Patients with stage IIIA/B unresectable nonsquamous non-small-cell lung cancer randomly received (1: 1) pemetrexed 500 mg/m2 and cisplatin 75 mg/m2 intravenously every 3 weeks for three cycles plus concurrent TRT (60 to 66 Gy) followed by pemetrexed consolidation every 3 weeks for four cycles (arm A), or standard therapy with etoposide 50 mg/m2 and cisplatin 50 mg/m2 intravenously, every 4 weeks for two cycles plus concurrent TRT (60 to 66 Gy) followed by two cycles of consolidation platinum-based doublet chemotherapy (arm B). The primary objective was OS. The study was designed as a superiority trial with 80% power to detect an OS hazard ratio of 0.74 with a type 1 error of .05. Results: Enrollment was stopped early because of futility. Five hundred ninety-eight patients were randomly assigned (301 to arm A, 297 to arm B) and 555 patients (283 in arm A, 272 in arm B) were treated. Arm A was not superior to arm B in terms of OS (hazard ratio, 0.98; 95% CI, 0.79 to 1.20; median, 26.8 v 25.0 months; P = .831). Arm A had a significantly lower incidence of any drug-related grade 3 to 4 adverse events (64.0% v 76.8%; P = .001), including neutropenia (24.4% v 44.5%; P < .001), during the overall treatment period. Conclusion: Pemetrexed-cisplatin combined with TRT followed by consolidation pemetrexed was not superior to standard chemoradiotherapy for stage III unresectable nonsquamous non-small-cell lung cancer.