Foreign body giant cell synovitis and focal bony destruction requiring secondary surgery developed in nine patients after carpal and radiocarpal arthroplasty with silicone rubber implants. Synovitis followed silicone rubber replacement of the lunate in four patients, the carpal scaphoid in two patients, and the trapezium in one patient, trapezial resurfacing hemiarthroplasty in one patient, and total wrist arthroplasty in one patient. All the removed implants were deformed. Some had changed in color from clear white at the time of insertion to yellow-white or deep yellow upon removal. Scanning electron microscopy of a silicone rubber implant of the lunate removed from one patient revealed extensive fibrillation in the capitate fossa. Light microscopy of the synovium revealed hyperplasia and hundreds of fragments of silicone particulate debris throughout the tissue adjacent to the implant. The silicone debris was surrounded by multinuclear foreign body giant cells in the eight patients from whom the synovium was studied. All patients required revision surgery. We believe that in the patients studied, synovitis represented a biologic reaction to the silicone fragments found scattered throughout the synovium for the following reasons: The synovitis found was not present before surgery (except in the patient with rheumatoid arthritis); the synovitis found was not associated with the condition for which replacement arthroplasty was performed (except in the patient with rheumatoid arthritis); synovitis subsided after implants were removed; and synovitis was noted histologically in areas of silicone debris and appeared directly related to silicone seeding. Fibrillation, fragmentation, and local seeding of silicone after carpal or radiocarpal arthroplasty appear related to gradual deformation of the implant, which was the result of repeated compressive loading and shearing. Clinically significant complications of rubber carpal or radiocarpal arthroplasty with silicone rubber implants may develop from silicone synovitis.