Abstract Background Immune checkpoint inhibitors (ICIs) can cause severe immune‐related adverse events (irAEs). However, biomarkers for irAEs common to different types of ICIs and cancers have not been reported. This study examined whether eosinophils can be used as a predictor of irAEs. Methods Six hundred fourteen patients with cancer (esophageal, gastric, head and neck, lung, melanoma, renal cell, urothelial, and other cancer) received anti‐PD‐1, anti‐PD‐L1, or anti‐CTLA‐4 plus anti‐PD‐1 therapy. The patients were divided into two groups depending on whether they experienced irAEs (irAE group) or not (non‐irAE group). Eosinophils were examined before the two‐course treatment. Results Patients in the irAE group who received anti‐PD‐1 or anti‐CTLA‐4 plus anti‐PD‐1 therapy had higher eosinophils before the two‐course treatment than those in the non‐irAE group (p