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There are few reports on the coexistence of cardiac amyloid light-chain (AL) amyloidosis and light chain deposition disease (LCDD), despite their similar pathophysiologies caused by plasma-cell dyscrasia. Herein, we report the coexistence of these diseases. A 59-year-old man was referred to our hospital because of exertional dyspnea and hypotension. Renal dysfunction of unknown etiology had been present for 4 years and hemodialysis had been introduced. Severe systolic and diastolic cardiac dysfunction was apparent, accompanied with dilatation and granular sparkling, but not with left ventricular hypertrophy. The plasma-free light chain κ was found to be extremely high, with a κ/λ ratio of 1,919. Light microscopic examination of the endomyocardial biopsy revealed spotty and homogenous deposits, which positively stained with Congo red, and exhibited a blazing apple-green color under polarized light. Based on these results, cardiac amyloidosis was diagnosed. In specimens prepared for electron microscopy, no amyloid fibrils could be found. Instead, we observed amorphous nonfibrillar deposits around several small vessels including capillaries and small arteries, which were consistent with light-chain deposits. LCDD was diagnosed based on the systemic increase in κ light chain and the ultrastructural findings of the endomyocardial biopsy specimens. Coexistence of cardiac amyloidosis and LCDD was thus confirmed in our patient. An electron microscopic assessment in addition to Congo red staining may be useful to diagnose latent LCDD in patients with suspected cardiac light-chain amyloidosis., (Copyright © 2020. Published by Elsevier Inc.)