Background: The tyrosine kinase inhibitor sorafenib is the standard of care for first-line HCC; currently, there is no clear standard of care after disease progression on sorafenib or for patients (pts) with intolerance to sorafenib. Because most HCC is driven by inflammation, there is a strong rationale to evaluate immunotherapy in pts with this type of cancer. The randomized, double-blind, placebo-controlled phase 3 KEYNOTE-240 study (ClinicalTrials.gov, NCT02702401) was designed to compare the efficacy and safety of the anti–PD-1 antibody pembro + BSC vs placebo + BSC in pts with previously treated advanced HCC. Methods: Eligibility criteria include age ≥ 18 years, histologically or cytologically confirmed diagnosis of HCC, documented progression after stopping treatment with sorafenib or intolerance to sorafenib, disease not amenable to a curative treatmentapproach (eg, transplantation, surgery, or ablation), measurable disease confirmed by central imaging vendor review per RECIST v1.1, Child-Pugh liver score A, ECOG performance status 0-1, and predicted life expectancy > 3 months. Pts will be randomly assigned 2:1 to receive pembro 200 mg IV Q3W + BSC or placebo Q3W + BSC for up to 35 cycles (~2 years) or until disease progression, unacceptable toxicity, or investigator decision. Randomization will be stratified by geographic region, presence of macrovascular invasion, and α-fetoprotein level. BSC will be provided by the investigator per local treatment practices. Response will be assessed every 6 weeks per RECIST v1.1 by central imaging vendor review. Adverse events (AEs) will be assessed throughout treatment and for 30 days thereafter (90 days for serious AEs) and graded per NCI CTCAE v4.0. Primary objectives are comparison of progression-free survival per RECIST v1.1 by central imaging vendor review and overall survival between treatment arms. Secondary objectives are comparison of objective response rate, duration of response, disease control rate, and time to progression per RECIST v1.1 by central imaging vendor review; and evaluation of safety and tolerability. Planned enrollment in KEYNOTE-240 is 408 pts across 26 countries. Clinical trial information: NCT02702401