Many studies have shown that in Africa, particularly in Togo, the 20- to 29-year-old age group is the age group most frequently affected by AIDS. This age group accounts for 84% of the students of the University of Benin. We studied students, most of the age group thought to be most at risk, investigating sexual behavior, knowledge and attitudes to AIDS and sexually transmitted diseases (STDs). The level of knowledge about the problems of AIDS and STDs was similar for both sexes and for all ages and levels of education of the students. Students had a reasonable knowledge of AIDS, particularly concerning the transmission of HIV (88.6% of students aware), risk behavior (80.8%), AIDS treatment (57.0%) and more general information about HIV (49.4%). They were poorly informed about the transmission (42.9%) and complications (0.69%) of other STDs. Most students had positive attitudes towards HIV issues, particularly the use of preventive measures (3.41 in 5) and the acceptance of infected individuals (3.98 in 5). However, few had seriously considered that AIDS and STDs might impact on their own sex lives (1. 84 in 5) and some were even fatalistic concerning HIV infection. The students were highly sexually active, having intercourse a mean of 31 times per year. Their sexual behavior depended on age and sex. The 15- to 19-year-olds preferred occasional partners. They had sexual intercourse 1 to 3 times per month and used condoms 10 to 20% of the time. The 20- to 29-year-olds had multiple partners. They had sexual intercourse 3 to 5 times per month and used condoms more than 30% of the time. Students over the age of 30 had many partners in addition to their regular partner. They had sexual intercourse 5 to 10 times per month and used condoms 0 to 20% of the time. Significantly more women than men had high-risk sexual behavior (40. 5% of men claimed to regularly use condoms, versus only 22.7% of the women and 11.9% of the women accepted anal penetration versus only 8. 4% of men). The general assumption is that students, who have a high level of education, should be well informed concerning AIDS and STDs and should therefore have positive attitudes and responsible sexual behavior. This study demonstrates that the assumption bears no resemblance to reality. The students were aware of the way in which HIV is transmitted and of what construes risky behavior, but there was nonetheless a high frequency of high-risk behavior (e.g. multiple sexual partners, anal and oral sex, homosexuality, intravenous drug use). The behavior of the students was not significantly different from that of young people living on the streets of Lomé. There were significant relationships between knowledge and attitudes and between some types of sexual behavior and knowledge. However, there was no correlation between attitudes and behavior. The education of young people should focus on both the prevention of AIDS and STDs and on facing up to these diseases.