Aim: To develop a comprehensive item library of patient-reported, immunotherapy-related adverse events (irAEs) that draws from and expands on the Functional Assessment of Chronic Illness Therapy (FACIT) Measurement System. Methods: Literature review and iterative expert input. Based on a literature review of irAEs, we developed a framework of immunotherapy classes and their associated symptoms. Clinical experts then reviewed iterations of symptom summaries and item maps linked to the immunotherapy framework. Experts provided content review and feedback was shared across experts until consensus was reached. The iterative process facilitated creation of a Primary Symptom List associated with immune checkpoint modulators (ICMs), drawn from the larger set of symptoms. Existing FACIT items were mapped to the symptom list, and new items were written as needed to create the item library.Results:ThefullitemlibraryofirAEsiscomprisedof239items,covering142uniquesymptomsacross75inflammatory reactions/immune conditions. A subset of 66 items comprises a Primary Symptom List considered most common/relevant to ICM treatment. This includes gastrointestinal, skin, pulmonary, neurologic, musculoskeletal, and multiple miscellaneous and constitutional symptoms.Conclusion: The FACIT Immunotherapy Item Library is a compilation of 239 self-report items that capture the wide range of AEs experienced by people receiving immune treatments. A subset of 66 items comprises a Primary Symptom List meant for ICM therapy. Use of items selected from this library is encouraged in clinical research and clinical practice evaluation.