The etiology of fibrous capsular contractures in patients with silicone prostheses is unclear. However, cellular responses to the silicone polymers of the prostheses have not been examined. The exposure of human dermal fibroblasts to the components of the silicone gel prosthesis results in a significant change in cellular configuration and a progressive reduction in cell proliferation as determined by total matrix protein assays and hemocytometer cell counts. Transmission electron microscopy, however, documents a twofold increase in the rough endoplasmic reticulum when cells are exposed to the silicone gel. These findings suggest significant alterations in the behavior of human fibroblast subpopulations in response to silicone polymers.