Thrombospondins (TSPs) are matricellular proteins involved in intimal hyperplasia (IH). We hypothesized that ) TSP-1, TSP-2, and TSP-5 are interdependent regarding their effects on vascular smooth muscle (VSMC) physiology; ) local or systemic knockout of or reduces IH, with its combination () being most effective; ) local or systemic knockout of increases IH; and ) the effects of TSPs differ between males and females. In vitro, VSMCs were transfected with siRNA against , , , or VSMC proliferation by TSP-1, TSP-2, or PDGF-BB was tested, and chemotaxis to TSP-1, TSP-2, TSP-5, or PDGF-BB was assessed. Sprague-Dawley male and female rats underwent carotid artery balloon injury with intraluminal treatment of saline or adeno-associated virus containing siRNA against , , , , or scrambled siRNA. Wild-type, , , or null male or female mice underwent carotid artery ligation. After 14 days (rat) or 28 days (mice), animals were perfusion-fixed, euthanized, and IH measured. In vitro, siRNA to , , , or decreased VSMC response to exogenous TSPs. The novel combined siRNA demonstrated the most robust decrease in proliferation and migration. In vivo, only male rats and mice had reduced IH with local or systemic knock down of or ( < 0.05), with combined siRNA to 1/2 having the most robust effect. Knockdown of increased IH only in female mice ( < 0.05). In conclusion, TSPs affect one another and demonstrate a sexual dimorphism that may explain differences between male and female IH. Thrombospondins (TSPs) are matricellular proteins involved in intimal hyperplasia (IH). We demonstrate in vitro, TSP-1, TSP-2, and TSP-5 affect one another and influence vascular smooth muscle cell proliferation and migration. In vivo, using a rat and mouse model of IH, we show that TSPs demonstrate a sexual dimorphism that may explain differences between male and female IH. Particularly, TSP-1 and TSP-2 appear to be strong mediators of IH in males only.