Objective: Immune checkpoint inhibitors (ICI) have significantly improved the treatment efficacy of a variety of malignant tumors. However, patients may experience a series of special side effects during treatments with ICI. Immune-related myositis after ICI treatment is characterized by autoimmune rheumatic and musculoskeletal damage, which is relatively rare. To analyze the clinical characteristics and outcomes of ICI-associated myositis in urological tumors, we summarized the clinical manifestations, electrophysiological and pathological characteristics, treatments and outcomes in 8 patients. Methods: The clinical data of the 8 patients with immune-related myositis after ICI treatment for urological tumors treated in the Department of Urology, Peking University First Hospital from March 2018 to March 2022 were retrospectively analyzed for demographic characteristics, drug regimen, clinical symptoms, laboratory indices, electromyography examination, pathological manifestations and outcomes. Results: The eight patients included 2 females and 6 males with a median age of 68 years, all treated with ICI for urological neoplasms, including 2 upper tract urothelial carcinoma (UTUC), 3 renal cell carcinoma (RCC), and 3 bladder cancer (BCa). The median time between the first ICI treatment and the detection of immune-related myositis was 39.5 days, and the median duration of treatment was 2 sessions. The main symptoms were muscle pain and weakness, 5 cases with ptosis, 3 cases with secondary rhabdomyolysis, 5 cases with myocarditis, 1 case with myasthenia gravis, and 1 case with enterocolitis. Among them, patients with immune-related myocarditis had a shorter interval from the first anti-programmed cell death protein-1 (PD-1) therapy to the onset of immune-related myositis (P=0.042) compared with patients without myocarditis. The 8 patients had significant elevation of transaminases and muscle enzyme profile indexes, and 5 patients showed positive auto-antibodies. 3 patients had perfected muscle biopsies and showed typical skeletal muscle inflammatory myopathy-like pathological changes with CD3⁺, CD4⁺, CD8⁺, CD20⁺ lymphocytes and CD68⁺ macrophage infiltration. After the diagnosis of immune-related myositis, all the 8 patients immediately discontinued ICI therapy and improved after intravenous administration of methylprednisolone alone or in combination with gamma-globulin. Conclusion: Immune-related myositis after ICI treatment is an immune-related adverse reactions (irAEs) with unique clinical and pathological features, commonly combined with cardiovascular adverse reactions. Immediate discontinuation of ICI and initiation of glucocorticoid therapy may improve the patient's condition in a timely manner.