Background: Standard therapy for pts with unresectable stage III NSCLC is concurrent platinum doublet chemotherapy with radiotherapy (CCRT); however, this therapy does not reduce the risk of distant relapse, and the 5-y survival rate is low. The anti–PD-1 checkpoint inhibitor pembrolizumab has durable clinical activity as first-line therapy for advanced/metastatic NSCLC: as monotherapy for PD-L1–positive tumors and in combination with chemotherapy irrespective of PD-L1 status. KEYNOTE-799 evaluates the safety/efficacy of first-line pembrolizumab plus CCRT for unresectable, locally advanced stage III NSCLC. Methods: This nonrandomized, open-label phase 2 study enrolls pts ≥18 y with previously untreated, unresectable, pathologically confirmed stage IIIA–C NSCLC with measurable disease per RECIST 1.1. Pts receive 17 cycles of pembrolizumab 200 mg Q3W plus standard thoracic radiotherapy in cycles 2 and 3 (60 Gy in 30 daily 2 Gy fractions). In cycles 1–3, treatment also includes investigator’s choice of either paclitaxel 200 mg/m² and carboplatin area under the curve (AUC) 6 Q3W for 1 cycle, followed by paclitaxel 45 mg/m² and carboplatin AUC 2 weekly for 6 weeks, or cisplatin 75 mg/m² and pemetrexed 500 mg/m² Q3W (nonsquamous only). Tumor imaging occurs at baseline and Q9W until week 54, verified PD, initiation of new cancer therapy, study withdrawal, or death. AEs are graded by NCI CTCAE v4.0. Primary endpoints are the rate of grade ≥3 pneumonitis and ORR (RECIST 1.1 by blinded independent central review [BICR]); confidence intervals for both will be estimated by the Clopper-Pearson method. Secondary endpoints are PFS (RECIST 1.1 modified to follow ≤10 target lesions; ≤5 per organ by BICR), OS, and safety. PFS and OS will be analyzed by Kaplan-Meier method. Approximately 216 pts (108 per cohort) will be enrolled in 59 sites in 10 countries beginning on Nov 5, 2018. As of Feb 12, 2019, 30 pts have enrolled. Continuous interim analyses using binomial sequential testing will be performed after ≥36 pts have ≥15 weeks of follow up in each cohort, to allow earlier treatment discontinuation, if required. Clinical trial information: NCT03631784