A 61-year-old man was referred to our hospital due to a 3-month history of fever of unknown origin, and with right knee and ankle joint pains. At another hospital, extensive investigations had produced negative results, including multiple sterile cultures of blood and joint fluids, and negative autoantibodies. His serum uric acid level was not elevated. However, after admission to our hospital, we performed right knee arthrocentesis, which revealed uric acid crystals. These findings, combined with the results of imaging tests, which showed joint degeneration, led to a diagnosis of advanced erosive gout. After receiving a therapeutic non-steroidal anti-inflammatory drug and a maintenance dose of colchicine for prophylaxis against recurrence, the patient's symptoms subsided and did not return. Advanced erosive gout should be considered a possible cause of fever of unknown origin and diagnostic arthrocentesis should be performed in patients with unexplained arthritis.