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Abstract Background Pseudoprogression, the initial apparent worsening of cancer prior to eventual improvement, is a documented feature of immune checkpoint inhibitor administration and presents a challenge to clinicians distinguishing true progression from pseudoprogression. This phenomenon does not typically occur with traditional cytotoxic chemotherapy. We present a case in which a patient treated with combination carboplatin-pemetrexed plus pembrolizumab experienced transient radiographic worsening of disease with subsequent regression. Case presentation A 65-year-old never-smoking white male with advanced sarcomatoid non-small cell lung cancer (NSCLC) harboring a MET exon 14 skipping mutation and with PD-L1 tumor proportion score of 80% was initiated on combination chemotherapy plus immune checkpoint inhibitor (ICI) therapy after progression on a MET inhibitor. After two cycles of carboplatin-pemetrexed plus pembrolizumab, repeat imaging suggested disease progression. Following discontinuation of the carboplatin-pemetrexed plus pembrolizumab regimen, the patient reported improved symptoms and energy levels, which were attributed to the waning of treatment-associated toxicities. On the day prior to initiation of the next planned line of therapy, repeat imaging was preformed to provide a baseline for treatment efficacy. Imaging revealed improvement compared to the prior imaging. Chemotherapy with carboplatin-pemetrexed plus pembrolizumab was resumed, with response ongoing 8 months later. Conclusions Pseudoprogression is a documented feature of ICI administration. Pseudoprogression is not typically observed in patients treated with traditional cytotoxic chemotherapy and has not yet been documented in patients treated with combination cytotoxic chemotherapy plus immunotherapy. At this time, there are no reliable means to predict or diagnose these rare events; therefore, more studies should be conducted to understand which patients are predisposed to developing this phenomenon and to increase clinical recognition.