Background: Results from cohort 1 of KEYNOTE-224, an open-label, single-arm, multi-country phase 2 trial, demonstrated that pembro monotherapy was efficacious and tolerable in patients (pts) with advanced HCC previously treated with sorafenib. Here, we report results from KEYNOTE-224 cohort 2, which enrolled pts with advanced HCC and no prior systemic therapy. Methods: Eligible pts in cohort 2 had radiologically, histologically, or cytologically confirmed, incurable HCC not amenable or refractory to locoregional therapy, Child Pugh A liver disease, measurable disease based on RECIST 1.1 by blinded independent central review (BICR), ECOG PS 0-1, and BCLC stage C or B. Pts received pembro 200 mg IV Q3W for ̃2 years or until disease progression, unacceptable toxicity, consent withdrawal, or investigator decision. Primary endpoint was ORR (RECIST 1.1 by BICR). Secondary endpoints included DOR, DCR, TTP, PFS, OS, and safety/tolerability. Response was assessed every 9 weeks. Efficacy and safety were assessed in pts who received ≥1 dose of study treatment. DOR was assessed in responders. The estimate and 95% CI of the ORR and DCR were based on the Clopper-Pearson method. Kaplan-Meier method was used to estimate OS, PFS, and DOR. A sample size of ̃50 pts was chosen to provide acceptable precision for the assessment of ORR. Results: Cohort 2 enrolled 51 pts. The median time from the first dose to data cutoff (July 31, 2020) was 21 (range, 17-23) mo. The median age of pts was 68 (range, 41-91) years, one pt was HBV+, 80% had alcohol use, 8% were HCV+, 18% had vascular invasion, 35% had extrahepatic disease, 33% had BCLC Stage B disease, and 67% had BCLC Stage C HCC. ORR was 16% (95% CI, 7-29) and was similar across most subgroups. Median DOR was not reached (range, 3-20+ mo); 70% were estimated to have response duration ≥12 mo. Best overall responses were 0 CR, 8 (16%) PRs, 21 (41%) SDs, and 17 (33%) PDs; response was not evaluable or not assessed for 5 (10%) pts. DCR was 57%. The median TTP was 4 (95% CI, 3-8) mo. The median PFS was 4 (95% CI, 2-6) mo, and median OS was 17 (95% CI, 8-NA) mo. PFS rate at 18 mo was 16%, and OS rate at 18 mo was 46%. Treatment-related AEs (TRAEs) occurred in 27 (53%) pts; the most common TRAEs were diarrhea, fatigue, hypothyroidism, and myalgia. Grade ≥3 TRAEs occurred in 7 (14%) pts. TRAEs led to treatment discontinuation in 6% of pts. Immune-mediated AEs and infusion reactions occurred in 11 (22%) pts. One treatment-related death occurred due to myocarditis, with associated immune-related hepatitis. Conclusions: In pts with advanced HCC and no prior systemic therapy, pembro monotherapy provided durable anti-tumor activity, promising overall survival, and demonstrated a safety profile consistent with that previously observed for pembro in advanced HCC. These findings support further evaluation of pembro-based regimens for the treatment of HCC in the frontline setting. Clinical trial information: NCT02702414