BACKGROUND: Lower capsular contracture rates have been observed with peri-prosthetic fat grafting. We investigated the effect of fat grafting on capsular characteristics and peri-prosthetic collagen density, content, and fiber alignment. METHODS: Forty miniature tissue expanders were placed on the backs of 20 rats. After four weeks, both inguinal fat pads were harvested, homogenized, and injected into peri-prosthetic tissue of the right tissue expander (fat graft) while the left served as control. The animals were killed at three (10 rats) and 12 weeks (10 rats), and full thickness peri-prosthetic samples were histologically processed for morphology (H&E) and collagen type and content (picrosirius red). RESULTS: An 8.1% increase in adipose peri-prosthetic thickness was associated with a 10% decrease in collagen content at any time point ( = 0.004). Fat-grafted capsules displayed a 59% reduction in % total collagen when compared with controls ( < 0.001). There were no differences in capsular thickness. Fat-grafted samples were 54 times more likely to have a higher inflammation score and 69 times more likely to have a lower capsular density score than their nongrafted counterparts ( 0.001 and 0.001, respectively). The extent of inflammation decreased over time in all samples ( 0.002). Additionally, fat-grafted samples were 67 times more likely to have a lower fiber alignment score than the controls ( 0.001). CONCLUSIONS: Enhancement of peri-prosthetic tissue with fat grafting decreases collagen content, density, and fiber alignment of implant capsules. These findings support clinical application of fat grafting in prosthetic breast surgery to potentially decrease capsular contracture.