There is an urgent need for a safe and effective vaccine to prevent human immunodeficiency virus (HIV) infection. Several HIV vaccine candidates have shown promise, but many concerns regarding the safety and efficacy of current vaccines remain. A major hindrance in HIV vaccine development is a poor understanding of precisely what functions HIV vaccines are required to perform in order to protect humans from HIV-1. Only higher primates (i.e. macaques, chimpanzees and humans) are susceptible to HIV-1 or the closely related virus 'simian immunodeficiency virus'. These species are outbred and there are remarkable genetic differences in both the immune responses to vaccines and their susceptibility to infection. The development of genetically identical macaques would be a major step towards dissecting what immune responses are required to protect from HIV infection. For example, live attenuated HIV-1 vaccines are likely to be highly efficacious, but will induce disease in a substantial proportion of recipients. Defining why a live attenuated vaccine is effective should allow safer vaccines to be developed, retaining only the immunologic properties of an effective vaccine. The reduction in 'background genetic noise' obtained by studying genetically identical primates would provide concise answers to critical HIV vaccine issues, by studying a minimal number of animals. Such an approach could potentially be employed in other diseases where non-human primates are the only available model. Small studies can be performed where identical twins are generated by embryo bisection; however, larger studies where multiple immune parameters are simultaneously evaluated would be facilitated by cloning technology. Despite the technical difficulties to be overcome, the potential gains in human health from the development of genetically identical non-human primates are worthy of careful consideration.