A man developed acute monoarticular ankle arthritis caused by calcium pyrophosphate dihydrate (CPPD) crystals. The clinical syndrome resembled that of a pyogenic arthritis. Synovial fluid analysis revealed a glucose concentration of 13 mg/dL and 99,000 white blood cells/mm3. Only one other report of an extremely low synovial fluid glucose associated with pseudogout could be found.The diagnosis of pseudogout was initially suggested when rhomboidal forms were seen during synovial fluid Gram stain examination. Synovial fluid examination with polarized microscopy was initially negative, but revealed numerous CPPD crystals when repeated on the third hospital day.This case serves to illustrate how pseudogout can mimic pyogenic arthritis in both clinical presentation and low synovial fluid glucose concentration. The examination of Gram-stained synovial fluid can reveal the rhomboidal forms of CPPD crystals. The appearance of these crystals is documented in this report.