The introduction of the hepatitis A virus into a population of non-immune young adults, whose sexual practices facilitate fecal-oral transmission of the virus, represents a situation with epidemic potential. Indeed, several hepatitis A outbreaks among gay men have been reported in various countries. The authors report here the results of an exploratory investigation conducted by the French Institute for Public Health Surveillance (InVS), in collaboration with the "REZO 85" network of private practitioners (a network of private practitioners, generalists, and dermatologists who provide care for a large number of male homosexual patients and treat patients infected with HIV) and the Department of Infectious and Tropical Diseases at Rothschild Hospital, conducted regarding clusters of hepatitis A cases that occurred in Paris. On June 15, 2000, the InVS was informed by a physician from the Infectious Diseases Department at Bichat Hospital of the occurrence of several cases of hepatitis A among HIV-infected gay men. An exploratory investigation was conducted to identify possible modes of transmission and to consider prevention strategies. This investigation also provided data to assess the relevance of extending official recommendations for hepatitis A vaccination to the population of gay men.