Nicotinamide adenine dinucleotide phosphate oxidases (NOX) are enzymes that generate reactive oxygen species (ROS). NOX2 activity in the vascular wall is elevated in hypercholesterolemia, and contributes to oxidative stress and atherogenesis. Here we examined the role of another NOX isoform, NOX1, in atherogenesis in apolipoprotein E-knockout (APOE(-/-)) mice fed a Western diet for 14 weeks. Although NOX1 mRNA expression was unchanged in aortas from APOE(-/-) versus wild-type mice, expression of the NOX1-specific organizer, NOXO1, was diminished, consistent with an overall reduction in NOX1 activity in APOE(-/-) mice. To examine the impact of a further reduction in NOX1 activity, APOE(-/-) mice were crossed with NOX1(-/y) mice to generate NOX1(-/y)/APOE(-/-) double-knockouts. NOX1 deficiency in APOE(-/-) mice was associated with 30-50 higher plasma very-low-density lipoprotein (VLDL)/LDL and triglyceride levels (P <0.01). Vascular ROS levels were also elevated by twofold in NOX1(-/y)/APOE(-/-) versus APOE(-/-) mice (P <0.05), despite no changes in expression of other NOX subunits. Although en face analysis of the descending aorta revealed no differences in plaque area between NOX1(-/y)/APOE(-/-) and APOE(-/-) mice, intimal thickening in the aortic sinus was increased by 40 (P <0.05) in the double-knockouts. Moreover, NOX1 deficiency was associated with a less stable plaque phenotype; aortic sinus lesions contained 60 less collagen (P <0.01), 40 less smooth muscle (P <0.01), and 2.5-fold higher levels of matrix metalloproteinase-9 (P <0.001) than lesions in APOE(-/-) mice. Thus, these data, which suggest a protective role for NOX1 against hyperlipidemia and atherosclerosis in APOE(-/-) mice, highlight the complex and contrasting roles of different NOX isoforms (e.g., NOX2 versus NOX1) in vascular pathology.