Background: The open-label, randomized, phase 3 KEYNOTE-040 study (NCT02252042) showed that pembro vs standard of care (SOC) chemotherapy prolonged survival in patients (pts) with recurrent and/or metastatic HNSCC whose disease progressed during/after platinum-based therapy. Post hoc analyses were conducted to evaluate pembro vs SOC by prior RT and disease state (metastatic, locoregionally recurrent [referred to as recurrent herein], or recurrent and metastatic [R/M]). Methods: Pts (N = 495) were randomly assigned (1:1) to receive pembro (200 mg Q3W) or investigator choice of methotrexate (40 mg/m² QW), docetaxel (75 mg/m² Q3W), or cetuximab (400 mg/m² loading dose, then 250 mg/m² QW). Primary end point was OS; PFS and ORR were secondary end points. Results: 175, 97, and 195 pts had metastatic, recurrent, and R/M disease, respectively (28 pts had unknown disease state); 64 pts had no prior RT; 431 pts had prior RT. As in the ITT population, prolonged survival benefit and trend toward improved PFS and ORR was observed with pembro vs SOC in pts with prior RT (Table), and prolonged survival benefit was observed with pembro vs SOC in pts with metastatic and R/M, but not recurrent, disease. Conclusions: In this post hoc analysis, patients with prior RT benefited from treatment with pembro vs SOC. For patients without RT, sample sizes are too small to draw any definitive conclusions. Survival benefit of pembro vs SOC was observed in pts with metastatic and R/M disease. Clinical trial information: NCT02252042