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To characterize the natural history of immunoglobulin light chain amyloidosis–associated myopathy and to provide guidelines for recognition.Fifty-one patients with systemic immunoglobulin light chain amyloidosis and biopsy-confirmed muscle amyloid deposition diagnosed between January 1, 1995, and December 31, 2015, were included in this study.Common presenting symptoms were muscle weakness in 49 patients (96%), dysphagia in 23 (45%), myalgia in 17 (33%), macroglossia in 17 (33%), jaw claudication in 13 (25%), and hoarseness in 9 (18%). The median time from the onset of symptoms to diagnosis was almost 2 years. Less than two-thirds of the patients with an outside muscle biopsy (16 of 27) had an established pathologic confirmation of amyloidosis due to failure to routinely incorporate Congo red staining. Moreover, 12 patients were incorrectly treated before diagnosis of amyloid myopathy. More than half of the patients had normal creatine kinase levels at diagnosis. Cardiac troponin T levels were elevated above the reference range in 5 of 12 patients who lacked evidence of cardiac involvement. Median overall survival was 32 months. Factors associated with inferior survival were involvement of more than 2 organs (median survival, 13 months), cardiac involvement (median survival, 15 months), and absence of stem cell transplant (median survival, 18 months). With the exclusion of patients treated with stem cell transplant, no improvement in survival was seen over the 1995-2004 and 2005-2015 decades.Immunoglobulin light chain amyloidosis–associated myopathy is rare. Delay in diagnosis is common, and there is a high rate of pathologic and clinical misdiagnosis. Awareness of elevation of cardiac troponin T levels in the absence of cardiac disease may be a clue to diagnosis.