Exposure to blood during sexual intercourse is potentially important and yet understudied in HIV transmission. The study included 415 men and 127 women receiving diagnostic and treatment services at a sexually transmitted infections (STI) clinic in Cape Town, South Africa, who completed anonymous sexual health behaviour surveys. More than 30% of both men and women reported engaging in sexual intercourse involving genital bleeding in the previous 3 months. Individuals who had engaged in sexual intercourse involving blood had significantly greater numbers of sex partners in the previous 3 months (OR = 1.6, 95%, CI = 1.1-2.3), engaged in higher rates of unprotected vaginal intercourse (OR = 1.2, 95%, CI = 1.1-1.2), were significantly more likely to have exchanged sex for money or materials (OR = 3.1, 95%, CI = 1.7-5.5) and were significantly more likely to have experienced condom breaks during intercourse (OR = 1.7, 95%, CI = 1.2-2.4). These findings suggest that exposure to blood during vaginal intercourse is prevalent among STI clinic patients in Cape Town and may be an important contributing factor to the rapid spread of HIV in South Africa.