Studies of heterosexual HIV transmission have consistently found anal intercourse to be a highly predictive risk factor for seroconversion. Yet most AIDS prevention messages targeted at heterosexuals, presumably influenced by cultural taboos against acknowledging this sexual practice, continue to emphasize vaginal and, increasingly, oral sex transmission. The health risks of anal sex appear to be severely underestimated by a substantial proportion of sexually active women and men in North and Latin America as well as parts of South Asia, Africa, and other regions. Among heterosexuals reported rates of condom use are nearly universally lower for anal than for vaginal intercourse. This review examines anal sex among the general population, including its prevalence in various world regions, related sociocultural factors, and other associated health problems including anorectal STDs, Hepatitis B infection, and HPV-related anal cancer in women. U.S. survey and other data suggest that, in terms of absolute numbers, approximately seven times more women than homosexual men engage in unprotected receptive anal intercourse. Research among higher risk subpopulations, including bisexual men, injecting drug users, female sex workers, inner-city adolescents, and serodiscordant heterosexual couples, indicates that persons particularly at risk of being infected by or transmitting HIV are also more likely to practice anal sex. Considering this finding, along with the much greater efficiency for HIV infection as well as lower rates of condom usage, a significant proportion of heterosexual transmission in some populations is due to anal intercourse. This typically stigmatized and hidden sexual practice must be given greater emphasis in AIDS/STD prevention, women's care, and other health promotion programs.