Because oxidative stress and inflammation are known to play important roles in the pathogenesis of cardiovascular events that occur most frequently in the morning, we studied the association between reactive oxygen species (ROS) formation by polymorphonuclear leukocytes (PMNs) or mononuclear cells (MNCs) and morning blood pressure (BP) rhythm. A total of 31 hypertensives in whom ambulatory BP monitoring was performed participated in this study. They were first divided into three groups according to their nocturnal BP rhythm (non-dippers, dippers and extreme dippers), and then into two groups according to their morning BP change (surge-type and sustained-type). ROS formation by PMNs and MNCs was measured by gated flow cytometry. C-reactive protein and traditional risk factors such as age, gender, body mass index, hemoglobin A1c, and total cholesterol were also measured. ROS formation by MNCs was significantly increased in extreme dippers (vs. dippers, p<0.05, n=11) and in morning BP surge-type hypertensives (vs. sustained-type, p<0.05, n=13). In patients who were both extreme dippers and morning BP surge-types, ROS formation by MNCs was significantly higher than that in other groups. These results suggest that both extreme dippers and morning BP surge-type hypertensives may suffer increased ROS formation by MNCs, and that increased ROS formation by MNCs may underlie morning strokes.