Since the beginning of the HIV epidemic in north America, the majority of HIV infections have occurred among men who engage in sexual relations with other men. As the HIV epidemic enters its third decade, gay and bisexual men continue to have among the highest rates of HIV infection. Previous studies have highlighted the decline in the incidence of HIV and risk behaviour among gay and bisexual men. However, several studies have suggested that young gay and bisexual men continue to engage in unprotected sexual behaviours and are at continued risk of HIV infection. Recent reports in the media and research literature have indicated an increase in the incidence of HIV among gay and bisexual individuals in many of the world's major cities. The purpose of this study was to determine trends in HIV incidence using data from a prospective cohort of young gay and bisexual men.