Pembrolizumab is a humanized anti-programmed cell death 1 antibody used for the therapy of several malignancies. While autoimmune adverse events are not uncommon with this agent, they are typically mild and self-limiting. Severe autoimmunity is rare but can be life-threatening. Herein, we describe a unique case of severe proximal muscle weakness and joint pain shortly after beginning therapy with pembrolizumab. Work-up revealed elevated pro-inflammatory markers leading to the diagnosis of polymyalgia rheumatica. Steroids allowed for resolution of the joint pain. We call for awareness of this rare autoimmune toxicity with pembrolizumab.
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