The idea that circumcision decreases the risk of sexual transmission of HIV was first proposed in the 1980s, at the time of the worldwide emergence of HIV infection. Many descriptive studies have subsequently been conducted to confirm this effect. Over the last two years, three experimental studies have provided scientific proof of the protective effect of circumcision, evaluated to be about 60%. These studies were recently validated by the WHO. The underlying mechanism of this protective effect remains unclear, but appears to be related more to the number of CD4+ lymphocytes on the mucosal surface of the prepuce in uncircumcised men than to keratinisation of the glans in circumcised men. Paradoxically, the practical implications are unclear, as large-scale prophylactic circumcision, depending on the country, would raise problems of acceptability, material feasibility and even efficacy if the population, considering itself to be protected, abandons conventional safe sex precautions which remain essential.