We still do not know why the HIV prevalence in southern and eastern Africa is an order of magnitude higher than anywhere else in the world. An article in this journal in 2007 argued that a key determinant was not so much the lifetime numbers of sexual partnerships, but rather the high proportion of these partnerships that are arranged concurrently. Concurrency has been associated with elevated rates of sexually transmitted infections (STIs) elsewhere, but this relationship has never been demonstrated in an African setting, where its effect is proposed to be greatest. We conducted a secondary data analysis from a representative survey of 14 - 25-year-olds living in Cape Town to test the hypothesis that concurrency is associated with self-reported symptoms of an STI. On logistic multiple regression analysis we found a modest but statistically significant relationship between self-reported STI symptoms and having had a partner who engaged in concurrency.