We herein report the case of a 66-year-old Japanese woman who was admitted to our hospital due to diarrhea and malaise. She had been diagnosed with autosomal dominant polycystic kidney disease (ADPKD) at 35 years of age and had suffered from recurrent cyst infections since that time. Antibiotic therapy combined with hepatic cyst drainage and cyst sclerosing therapy led to transient improvements each time. At 66 years of age, watery diarrhea occurred. The patient's serum albumin level declined

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... 1.8 g/dL, and her C-reactive protein level was 4.5 mg/dL. An endoscopic biopsy of the descending colon revealed amorphous deposits in the small arteries and tissues of the submucosal layer. The deposits were positive for Congo Red staining and amyloid A staining. Therefore, AA-amyloidosis was diagnosed. An endoscopic biopsy of the stomach and duodenum also showed AA-amyloid deposits. If an ADPKD patient with a long history of cyst infection develops diarrhea and malaise, AA-amyloidosis should be considered as a possible complication.