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Myositis and myasteniform syndrome related to pembrolizumab. This case report concerns a 63-year-old man affected by metastatic undifferentiated liposarcoma. After receiving pembrolizumab as a second-line treatment in a clinical trial, the patient experienced an immune-mediated myocarditis, myositis and myasteniform syndrome. The last two adverse events showed significant clinical relevance in terms of severity, duration and the required specific treatment.Initial treatment approach consisted in pulses of 1 g of methylprednisolone, followed by 2 mg/kg/day, with clinical improvement. After 12 days, the immune-mediated myasteniform syndrome worsened, with dysphagia, dysphonia, bilateral palpebral ptosis and respiratory difficulty. Due to the refractoriness to glucocorticoid treatment, it was decided to initiate intravenous immunoglobulin at 2 g/kg, followed by 2 mg/kg every 4 weeks once discharged and mycophenolate 500 mg/12 hours, in order to reduce the dose of glucocorticoids.After 2 months, the patient presented an optimal clinical evolution, without muscular weakness and referred to an improvement in dysphagia and speech.