The efficacy of immunotherapies that use antibodies to block programmed cell death 1 (PD-1) has been extensively investigated for lung cancer. Along with reactivation of the patient's immune response to tumour cells, immune-related adverse effects with anti-PD1 therapy have been reported. We report an 80-year-old woman who had suffered from a primary lung adenocarcinoma pre-treated with pembrolizumab and was admitted to our hospital with serious autoimmune-mediated thrombocytopenia induced by pembrolizumab.