This work presents the results of the behavioural and serologic survey on HIV/AIDS conducted from December 2007 to December 2008 among the group of STD (sexually transmitted disease) patients, supposed to be at HIV infection risk. In Mauritania, the last survey of HIV seroprevalence among the STD patients goes up to the year 1995 (the prevalence was estimated then to be 0.9%). The goal was to determine the seroprevalence of HIV and syphilis and to gather information on the knowledge, the sexual behaviours on a risk concerning the HIV/AIDS, and the sexually transmitted disease among these patients. The census has been made on over 224 STD patients during the period of the study, without predominance of sex and with a majority of young adults. The prevalence for HIV is 9% and for the syphilis it is 10%. Actually, the condom is widely underused by this group, even in occasional intercourse. The STD patients are a group of risk towards HIV, because of their risk behaviours and low level of knowledge.