Novel antineoplastic therapies are focused on harnessing our own immune system to fight cancer. To that end, cytotoxic T-lymphocyte-associated antigen 4 and programmed death ligand 1 are 2 coinhibitory signals that play central roles in decreasing T-cell response and represent a class of medications termed "checkpoint inhibitors." We present an unusual case of progressive conduction abnormalities induced by checkpoint inhibitors. Prompt medical intervention resulted in full recovery. Despite the anticancer efficacy, the newer antineoplastic agents pose a significant and often life-threatening risk of cardiotoxicity.