A study was conducted to investigate the effect of in vivo aging on the physical, mechanical, and chemical properties of Silastic II gel-filled breast implants. In the study, the properties of 16 Silastic II gel-filled explants (retrieved from eight patients), with in vivo duration times ranging from 4 months to 13 years, were compared with lot-matched control (unimplanted) samples. Tensile and tear strength properties were measured for both explant and control shells by using identical testing protocols. The tensile strength properties of shells, which were extracted with hexane to remove non-cross-linked silicones, were also measured. Swelling measurements were used to determine the average molecular weight between cross-links (or entanglements). In addition, scanning electron microscopy was applied in the comparison of the morphological features of the explants and their lot-matched controls. The results of the study suggest that the silicone polymer used to fabricate the shells does not undergo appreciable degradation for up to 13 years in vivo. The study represents an investigation of the world's largest known inventory of explanted breast implants with lot-matched controls.