Background: The PACIFIC study showed that consolidative Programmed Death Ligand 1 inhibition (PD-L1i) after chemoradiation therapy (CRT) improves PFS and OS in patients with Stage IIII NSCLC (Antonia et al. NEJM 2017, 2018). Limited data, however, exist regarding the incorporation of PD-L1i concurrently during CRT. We sought to assess the safety and toxicity of PD-1i using pembrolizumab (pembro) during definitive CRT for Stage III NSCLC. Methods: In this multi-center prospective Phase I clinical trial using a 3+3 design, we evaluated the timing and dosing of pembro combined with chemotherapy (carboplatin + paclitaxel weekly) and definitive RT (60 Gy in 2 Gy/day x 30 fractions) for unresectable, locally advanced Stage III NSCLC (AJCC 7^thEd). Dose Cohorts (C) evaluated were--C1: full dose (FD) pembro (200 mg IV Q3 weeks) 2-6 weeks after CRT; C2: reduced dose (RD) pembro (100 mg IV Q3 weeks) starting Day 29 of CRT; C3: FD pembro starting Day 29 of CRT; C4: RD pembro starting on Day 1 of CRT; C5: FD pembro starting on Day 1 of CRT. For each cohort, pembro was continued Q3 weeks for up to 18 cycles (as monotherapy after CRT in either RD or FD based on initial dose assignment). Dose Limiting Toxicity (DLT) was defined as Grade ≥4 pneumonitis within 21 days of cycle 1 of pembro. Results: We enrolled 23 subjects from 8/2016-11/2018; median follow up (f/u) was 11.4 mo (range, 3.1 mo- 25.2 mo). Median age was 69 yrs (range 53-85); 52% were women. No DLTs were observed in any of the cohorts (C1 to C5). Grade ≥3 immune-related adverse events (irAE) occurred in 4 patients (18%). irAE’s included: Grade 5 (bilateral), 3, 2 pneumonitis (n=1, 1, 4, respectively (6 total)); Grade 3 hyperglycemia (n=1); Grade 3 interstitial nephritis (n=1); Grade 2 thyroiditis (n=4); Grade 2 myositis (n=1); Grade 1-2 transaminitis (n=3). Median PFS for patients who received ≥2 doses (n=18) of pembro was 20.3 mo. Conclusions: Combined treatment with PD-Li and CRT for stage III NSCLC was well tolerated with promising PFS to date but showed an increased risk for irAEs, particularly pneumonitis. Based on these encouraging results, further prospective study of PD-1i and CRT for Stage III NSCLC is warranted. Clinical trial information: NCT02621398