Vascular COX-2 restrains atherogenesis. Aortic atherosclerotic lesion burden, represented by the percentage of lesion area to total aortic area, was quantified by en face analysis of aortas from male mice fed a HFD for 3 and of female mice at 6 months. Representative en face preparations are shown (Lower panels). Lesion area tended to increase in male or female COX-2 mutants fed a HFD for 3 or 6 months respectively (A and B). One-way ANOVA revealed a significant effect of genotype (male, P= 0.0001 and female, P= 0.003) on lesion progression. Holm Sidak’s multiple comparison tests were used to test significant differences between WT and COX-2 KOs. Data are means ± SEMs. **p< 0.01, ****p<0.0001; n=18-22 per genotype.