Increased reactive oxygen species (ROS) are implicated in several vascular pathologies associated with vascular smooth muscle hypertrophy. In the current studies, we utilized transgenic (Tg) mice (Tg^p22smc) that overexpress the p22^phox subunit of NAD(P)H oxidase selectively in smooth muscle. These mice have a twofold increase in aortic p22^phox expression and H₂O₂ production and thus provide an excellent in vivo model in which to assess the effects of increased ROS generation on vascular smooth muscle cell (VSMC) function. We tested the hypothesis that overexpression of VSMC p22^phox potentiates angiotensin II (ANG II)-induced vascular hypertrophy. Male Tg^p22smc mice and negative littermate controls were infused with either ANG II or saline for 13 days. Baseline blood pressure was not different between control and Tg^p22smc mice. ANG II significantly increased blood pressure in both groups, with this increase being slightly exacerbated in the Tg^p22smc mice. Baseline aortic wall thickness and cross-sectional wall area were not different between control and Tg^p22smc mice. Importantly, the ANG II-induced increase in both parameters was significantly greater in the Tg^p22smc mice compared with control mice. To confirm that this potentiation of vascular hypertrophy was due to increased ROS levels, additional groups of mice were coinfused with ebselen. This treatment prevented the exacerbation of hypertrophy in Tg^p22smc mice receiving ANG II. These data suggest that although increased availability of NAD(P)H oxidase-derived ROS is not a sufficient stimulus for hypertrophy, it does potentiate ANG II-induced vascular hypertrophy, making ROS an excellent target for intervention aimed at reducing medial thickening in vivo.