An 80-year-old man presented with an acute, rapidly worsening synovitis with a hot effusion of the right knee approximately 48 hours following a urologic procedure. Initial aspirates failed to reveal crystals and suggested an acute infectious arthritis. Because his course continued to deteriorate despite adequate antibiotic treatment, the patient underwent arthroscopy in order to place tubes and to effect adequate irrigation and debridement. The preoperative diagnosis was septic arthritis. At the time of arthroscopy, there was gross evidence of crystals that were subsequently diagnosed as calcium pyrophosphate. This case illustrates the importance of an adequate differential diagnosis and the usefulness of the arthroscope in obtaining adequate tissue and aspirate under confusing circumstances.