AbstractAimImmune checkpoint inhibitors (ICIs) have proven to be effective treatments for various cancers, but can also elicit immune‐related adverse events (irAEs). Given that severe irAEs can be life‐threatening, biomarkers that can predict the occurrence of irAEs are of paramount importance. ICIs affect the dynamics of lymphocytes, and alterations in these dynamics may play a role in the development and severity of irAEs. The aim of this study was to investigate the correlation between irAEs and changes in lymphocyte counts.MethodsInformation on irAEs was collected from 226 ICI cases from 2014 to 2020. We compared lymphocyte counts before treatment and at the onset of irAE and investigated the association between lymphocyte count fluctuations and the presence and severity of irAE, the course after steroid treatment, and overall survival.ResultsOf the 226 cases, 27 patients developed grade 3 or higher irAE. Compared to the other groups, the lymphocyte count in this group was significantly decreased at the time of irAE (p< 0.01). There was a trend toward a rapid increase in lymphocyte count in the steroid responder group compared to the non‐responder group. Regarding overall survival, patients with irAE had significantly longer survival than those without irAE (p= 0.0025). However, there was no association between changes in lymphocyte count and survival in patients with irAE.ConclusionThe percentage change in lymphocyte count was found to correlate with the incidence of severe irAEs. Close monitoring of the patient's condition is crucial when the lymphocyte count decreases during ICI treatment.