Many mechanisms, including oxidative stress, contribute to hypertension. This study investigated the possible associations between oxidative stress, blood pressure and arterial stiffness in black South Africans. Ambulatory blood pressure measurements were taken for 101 black South African men and 99 women. The stiffness indices included ambulatory arterial stiffness index (AASI) and pulse pressure (PP). Reactive oxygen species (ROS) levels (Po0.0001) were higher in the African women compared with men. ROS levels were also higher in hypertensive compared with normotensive men. The 24h systolic blood pressure (SBP; Po0.01), 24 h diastolic blood pressure (DBP; Po0.0001) and pulse wave velocity (PWV; Po0.01) were significantly higher in African men compared with women. There were unadjusted positive associations of 24 h SBP (r¼0.33; P¼0.001), 24 h DBP (r¼0.26; P¼0.008) and 24 h PP (r¼0.29; P¼0.003) with ROS in African men only. A positive association between AASI and ROS existed only in hypertensive men (r¼0.27; P¼0.035), but became nonsignificant (B¼0.0014; P¼0.14) after adjustments. Adjusted, positive associations of 24h SBP (B¼0.181; P¼0.018) and 24h PP (B¼0.086; P¼0.050) with ROS were again only evident in African men. ROS is positively associated with SBP and PP in African men, suggesting that increased ROS levels may contribute to hypertension in this population group. ; http://www.nature.com/jhh/journal/v26/n2/pdf/jhh2010134a.pdf ; http://dx.doi.org/10.1038/jhh.2010.134