Objective: To report the clinicopathological features and outcome of myositis in patients treated with immune checkpoint inhibitors (irMyositis). Methods: We retrospectively analyzed patients diagnosed with irMyositis in tertiary centers in Paris, France, and Berlin, Germany, from January 2015, to July 2017. Main outcomes were clinical manifestations and muscle histology, which included MHC-I, C5b-9, CD3, CD4, CD8, CD20, CD68, PD-1, PD-L1 and PD-L2 immunohistochemical stains. Results: Ten patients with metastatic cancer were included; median age was 73 years (range, 56-87). Median follow-up duration was 48 weeks (range 16-88). Six patients developed myositis during nivolumab therapy, one during pembrolizumab, one during durvalumab, and two during combined nivolumab and ipilimumab. Median delay between immune checkpoint inhibitor (ICI) initiation and myositis onset was 25 days (range, 5-87). Clinical manifestations were dominated by acute or subacute myalgia (8 patients), limb-girdle (7), axial (7) and oculomotor (7) weakness. Four patients had evidence of myocarditis. In all patients, CK levels were elevated (median 2668 U/L, range 1059-16620), while anti-acetylcholine receptor and myositis-associated antibodies were negative. Electrodiagnostic studies showed myopathic process without decrement in all patients. Muscle biopsy constantly showed multifocal necrotic myofibers, sarcolemmal MHC-I and endomysial inflammation, mainly consisting of CD68+ cells expressing PD-L1 and CD8+ cells expressing PD-1. ICI treatment was withdrawn in all patients; nine patients received immunosuppressive therapy, which consistently led to marked clinical improvement. Conclusions: irMyositis presents with remarkably homogeneous and unique clinicopathological features, expanding the nosological spectrum of inflammatory myopathies in patients with cancer. ICI withdrawal and treatment with corticosteroids improve outcome.