INTRODUCTIONImmune checkpoint inhibitors (ICIs) are a key component of standard anticancer systemic therapy. While their immune-related adverse effects (irAEs) have been widely described, there are few data on grade≥3 irAEs. The primary aim of our descriptive study was to evaluate their incidence and characteristics.METHODSAn observational, retrospective, monocentric study was conducted. It included patients with locally advanced or metastatic melanoma, non-small cell lung cancer or renal cell carcinoma who initiated ICI therapy between 2016-2021 and experienced at least one grade≥3 irAEs coded according to the MedDRA® system.RESULTSAll cancer types and ICIs combined, the incidence of grade≥3 irAEs was estimated at 11.7% [9.6-13.9]. These were mainly hepatobiliary (22%), gastrointestinal (17%), musculoskeletal (16%) and respiratory (16%) disorders. They occurred on average 6.2±6.2 months after the start of treatment, resulting in hospitalization or prolonged hospitalization in over 40 and 20% of cases, respectively. Resolution without sequelae was observed in 56% of cases, but four patients died.DISCUSSIONThis real-world study investigated three cancers and several ICIs, unlike previously published studies that focused on a single cancer and/or one ICI. It provides a better understanding of grade≥3 irAEs, most of which are reversible, which an aim to optimize patient care.