Little is known about the induction of acute calcium pyrophosphate dihydrate arthritis after the intra-articular injection of hylan G-F 20 (Synvisc). Two reports have documented this adverse effect after the intra-articular injection of hyaluronan. Our patient, a 60-year-old man with osteoarthritis in both knees, presented with a history of an arthroscopy with meniscus shaving 7 years previously. He was given an injection of hylan G-F 20 in the right knee joint. Two days after the second injection, pain and swelling of the knee occurred. There was a severe loss of physical function. Systemic inflammatory reactions such as fever were not observed. A microscopic investigation of the synovial fluid showed evidence of calcium pyrophosphate dihydrate crystals. Bacterial contamination was not detected. There was no indication for calcium pyrophosphate dihydrate in the history of the patient. Some days after receiving nonsteroidal anti-inflammatory drugs and an intra-articular injection of steroids, the symptoms disappeared.