Standard treatment for newly diagnosed patients with limited-stage small-cell lung cancer (SCLC) was concurrent chemoradiotherapy (CCRT). However, even if an initial response was obtained, the prognosis remained poor due to the aggressiveness and high probability of progression or relapse of SCLC. Therefore, this population had an urgent unmet clinical need. KEYLYNK-013 was a multicenter, phase 3, randomized, placebo-controlled, double-blind research evaluating the addition of pembrolizumab to CCRT followed by pembrolizumab with or without olaparib in previously untreated limited-stage SCLC patients. Eligible patients aged more than or equal to 18 years with newly diagnosed, pathologically confirmed, limited-stage (i.e., stage I-III) SCLC would be randomized 1:1:1 to CCRT plus pembrolizumab (Groups A and B) or CCRT plus placebo (Group C). Participants would receive pembrolizumab plus placebo (Group A), pembrolizumab plus olaparib (Group B), or placebo in the absence of illness progression (Group C). Dual primary objectives were progression-free survival according to RECIST version 1.1 by blinded central independent review and overall survival. Enrollment began in December 2020 at roughly 150 sites and continues ongoing. KEYLYNK-013 would offer important data regarding the efficacy and safety of pembrolizumab plus CCRT and pembrolizumab with or without olaparib post-CCRT in people with limited-stage SCLC.