To better identify patients at high risk for cardiovascular events, several markers of risk have been proposed for use in screening. Recently, oxidative stress and inflammation have been evaluated as potential tools for prediction of the risk of cardiovascular events. Among them, we have measured reactive oxygen species (ROS) formation by polymorphonuclear cells (PMNs) and mononuclear cells (MNCs), since they may be a possible link between inflammation and oxidative stress. ROS formation by PMNs and MNCs was measured by a gated flow cytometric assay. Such biotechnological method of measuring ROS formation by PMNs and MNCs will make it possible that we measure vascular oxidative stress and vascular inflammation at the same time from only small amount of blood. We will state in this review that ROS formation by PMNs and MNCs are regulated by different mechanisms, although PMNs and MNCs are circulating in the same blood. Moreover, we will state that ROS formation by PMNs are regulated by blood pressure, Hb A 1C and oxidided LDL. ROS formation by MNCs are regulated by vascular inflammation, and that ROS formation by MNCs are also related to various cardiovascular risks such as LV mass, norepinephrine, IMT, and nocturnal blood pressure.