Background: Across cancer types and indications, the use of ICIs is increasing. Guidelines indicate that most irAEs occur in the months immediately following receipt of ICI. Little is known about the frequency of delayed irAEs following ICI administration. Methods: Patients who received ICIs and were hospitalized following ICI initiation between January 2011 and October 2022 were identified using pharmacy and hospital admission databases. Patient demographics, and oncologic and irAE history were extracted from the medical record. All admissions were reviewed by two physicians to evaluate for irAEs, which were defined as suspected or confirmed using published disease-specific criteria. We examined the number of irAEs hospitalizations at < 6 months, 6-12 months, 1-2 years, 2-5 years, and >5 years from the first dose of the most recent line of ICI to date of admission. Results: Among the 930 patients hospitalized with irAEs, average age was 66.0 years (range 18.3 - 96.2) and 61.2% were male. Most common tumor types were melanoma (42.0%), lung (24.1%), and gastrointestinal (12.7%). Median time from start of last line of ICI to hospitalization was 98 days (IQR 37-244.3). One hundred sixty-six patients (19.4%) had been treated with >1 prior line of ICI and 102 (11.0%) were diagnosed with multiple irAEs during the same admission. Of all patients admitted, 147 (15.7%) presented 6 months – 1 year after start of ICI line, 79 (8.4%) presented after 1-2 years, 35 (3.7%) presented after 2-5 years, and 2 (0.2%) presented after >5 years. Rates of irAEs by time from start of ICI line are demonstrated. Conclusions: Although irAEs occur in a time-dependent fashion, delayed, serious irAEs in hospitalized patients are common (28% at 6+ months), with a subset presenting two years or more after start of ICI. Providers must remain vigilant for potential irAEs regardless of elapsed time from ICI, especially as patients live longer and ICIs become widely used in different settings. Future work is needed to define potential triggers of and patients at risk for delayed irAEs.