The potential public health impact of imperfect human immunodeficiency virus (HIV) type 1 vaccines was examined by use of deterministic mathematical models of virus transmission. Imperfect vaccines are defined as those that act to favorably alter the typical clinical course of disease in those immunized who acquire infection. The properties examined include a lengthened incubation period; reduced virus load, which acts to lower infectiousness; reduced susceptibility on exposure to infection; and an increase in risk behaviors by those vaccinated. Analyses suggest that, although imperfect vaccines would struggle to block transmission via cohort vaccination of those entering the sexually active age classes, they could have a substantial public health impact, as measured by reduced prevalence and mortality induced by acquired immunodeficiency syndrome (AIDS), provided the case reproductive number of HIV-1 among vaccinated individuals (R(0v)) was less than that among unvaccinated individuals (R(0)). This requires that any lengthening in the incubation period and, hence, the time period over which an infected vaccine recipient can transmit to susceptible sex partners, as well as any increase in risk behaviors, are more than offset by other effects, such as reduced susceptibility to infection and reduced infectiousness. Numerical studies based on a more complex model, which included representation of age, sex, heterogeneity in sexual activity, variable infectiousness, and different mixing patterns between risk groups, were used to confirm the general insights gained from a simple deterministic model.