The present report is based on assessment of 48 patients who underwent carpal Silastic H.P. implant arthroplasty (trapezium, condylar, scaphoid, lunate and scapholunate implants). Mean follow-up period was 29 months (range 6-82). Recurrent pain and/or evidence of wrist synovitis and lytic lesions made early subsequent surgery necessary in 10 patients. In the remaining 38 patients the post-operative course was followed for an average period of 33 months (range 8-82). Severe giant-cell silicone synovitis combined with isolated or disseminated osteolytic lesions were found in 17/30 (56%) patients with scaphoid or lunate implants and in 2/18 (11%) with other types of Silastic carpal implants. Well-defined cysts were observed within 8 months of insertion of the implant. Morphologically, an erosive giant-cell synovitis was regularly seen, with large quantities of intra- and extracellular silicone debris. Abraded material was also observed in central parts of normal bone and in lymph node tissue distant from the implant. The ultimate tissue response to this propagation of silicone particles is unknown. The situation is of great concern and the continued use of proximal carpal Silastic H.P. implants should at present be seriously questioned.