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Anti–programmed cell death protein 1 (PD-1) therapy can lead to unconventional tumor responses, including radiologic pseudoprogression. Here we have determined the real-world incidence of radiologic pseudoprogression in advanced NSCLC and compared radiologic response criteria for assessment of disease response.The electronic medical records of all patients with NSCLC who were receiving anti–PD-1 therapy at our institution over a 3-year period were retrospectively reviewed, and patients with clinically suspected radiologic pseudoprogression were identified. Patients without available follow-up imaging or clinical data were excluded. Imaging examinations were then analyzed to determine whether progression was confirmed on subsequent reimaging. Assessments of tumor response by the Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1 (RECIST 1.1), the unidimensional immune-related response criteria (iRRC), and the iRECIST criteria for all patients were performed and compared.A total of 228 consecutive patients began receiving anti–PD-1 therapy over a 3-year period. Of the 166 of these patients who were evaluable, most (80%) received nivolumab. Fifteen patients (9%) were clinically suspected of having radiologic pseudoprogression on account of tumor enlargement and/or development of new lesions on computed tomography images during the first 4 to 6 weeks of therapy, and they continued receiving anti–PD-1 therapy. Three of these patients (2%) demonstrated evidence of radiologic pseudoprogression at first reimaging. The iRRC and immune RECIST criteria were more accurate in classifying radiologic pseudoprogression as nonprogression; none of the three cases were deemed progression by the iRRC or immune RECIST, whereas all three cases were determined to be progression according to the Response Evaluation Criteria in Solid Tumors, version 1.1.Radiologic pseudoprogression is a clinical challenge but an uncommon occurrence in patients with NSCLC who are receiving anti–PD-1 therapy.