In low socio-economic status communities in South Africa, African men showed a low BMI. Data on the effect of low BMI on cardiovascular function are scant. The present study aimed to assess the associations between low BMI and markers of cardiovascular function such as pulse wave velocity (PWV) and blood pressure in Africans aged 35–65 years, with low socio-economic status. The study population (n 496) was stratified into a low-BMI group with BMI ≤ 20 kg/m2 and a normal-BMI group with BMI >20 kg/m2 and ≤ 25 kg/m2. Blood pressure (Omron HEM-757) and PWV (Complior SP; Artech-Medical) was determined. Africans with low BMI showed an increased arterial stiffness with significantly higher PWV compared with the normal-BMI group (men: P= 0·001; women: P= 0·026), which remained after adjustment. In men with low BMI, PWV correlated negatively with BMI before (r − 0·204; P= 0·012) and after (r − 0·200; P= 0·020) adjustment. Forward stepwise regression analyses indicated a negative association between PWV and BMI in African men. A J-curve was evident suggesting a detrimental effect of low BMI on cardiovascular function in Africans. A low BMI may contribute to the high prevalence of cardiovascular-related morbidity and mortality within a developing country.