Background: Approximately half of patients (pts) with HNSCC are diagnosed with locally advanced disease and treated with surgery or concomitant chemoradiotherapy (CRT) with cisplatin. Unfortunately, disease recurs in 40% to 60% of patients. The PD-l inhibitor pembro is approved for recurrent/metastatic HNSCC. CRT has immunomodulatory effects; preclinical data suggest that efficacy can be improved by adding pembro. The phase 3 KEYNOTE-412 trial (NCT03040999) will explore if CRT + pembro can improve outcomes of pts with LA-HNSCC. Methods: Adult pts with newly diagnosed, pathologically proven, treatment-naive LA-HNSCC will be enrolled. Study population will include p16-negative HNSCC (any T3-T4 or any N2a-N3 [AJCC 7th edition]) and p16-positive oropharyngeal cancer (any T4 or any N3). Other eligibility criteria: measurable disease per RECIST v1.1 by blinded independent central review (BICR), provision of tumor sample for biomarker analyses, ECOG PS 0 or 1, and eligible for definitive CRT but not considered for primary surgery. Pts will be randomly assigned 1:1 to receive either pembro 200 mg every 3 weeks (Q3W) plus CRT, which includes radiotherapy (RT; accelerated [70 Gy, six 2 Gy fractions/wk] or standard [70 Gy, five 2 Gy fractions/week] fractionation) plus cisplatin 100 mg/m² Q3W for 3 cycles only, or placebo Q3W plus CRT. Pts will be stratified by RT regimen, tumor site/p16 status, and disease stage. Treatment will continue until centrally confirmed disease progression, unacceptable AEs, decision to withdraw by pt or investigator, or completion of 17 doses of pembro/placebo. Disease status will be evaluated by CT or MRI 12 weeks after end of CRT, every 4 months during the next 2 years, and then every 6 months during years 3-5. Pts will be evaluated for neck dissection at 12 weeks after CRT. AEs will be monitored and graded using CTCAE v4.0 throughout the trial and for 30 days (90 days for serious AEs) after end of treatment. Primary efficacy end point is event-free survival per RECIST v1.1 by BICR. Key secondary end points: overall survival, quality of life, and safety and tolerability of pembro. Approximately 780 pts will be enrolled. Clinical trial information: NCT03040999