A 58-year-old woman with stage 4 adenocarcinoma of the lung being treated with pembrolizumab developed dyspnea, non-productive cough, and a right middle lobe infiltrate. Complete resolution of the infiltrate with cessation of pembrolizumab, initiation of prednisone and no antibiotic therapy suggested drug-associated lung toxicity as the cause. While the programmed death-1 (PD-1) inhibitors -pembrolizumab and nivolumab - have been implicated as a cause of diffuse or multifocal pulmonary infiltrates, the current case represents, to our knowledge, the first instance of a unilobar, focal infiltrate associated with their use. We speculate that the blockade of immune tolerance that is the hallmark of PD-1 inhibitors might cause atypical inflammatory reactions such as the focal lobar infiltrate seen in the current patient. Awareness of this novel radiographic pattern of drug-associated lung toxicity may enhance clinicians' management of patients receiving.