Religious constraints on sexuality may have consequences for the transmission of sexually transmitted diseases. Recognising that several Islamic tenets may have the effect, if followed, of reducing the sexual transmission of HIV, this paper tests the hypothesis that Muslims have lower HIV prevalence than non-Muslims. Among 38 sub-Saharan African countries, the percentage of Muslims within countries negatively predicted HIV prevalence. A survey of published journal articles containing data on HIV prevalence and religious affiliation showed that six of seven such studies indicated a negative relationship between HIV prevalence and being Muslim. Additional studies on the relationship of risk factors to HIV prevalence gave mixed evidence with respect to following Islamic sexual codes (e.g., vs. extramarital affairs) and other factors, but that benefits arising from circumcision may help account for lower HIV prevalence among Muslims.