Nonhuman primates are excellent animal models for human diseases because of their close relationship to humans. Indeed, comparisons of the chromosomes and DNA homologies between primates and humans testify to the commonality of the genetic material between these phylogenetically related species. Not surprisingly, this close relationship at the genotypic level extends to the phenotypic level. Thus, the patho-physiological responses of humans and nonhuman primates to internal and external insults are remarkably similar. Two types of human diseases for which nonhuman primates are paramount animal models are discussed. One type includes diseases with defined, single agent etiologies and to which all members of the species are genetically susceptible. Examples of these are leprosy, AIDS, hepatitis and Parkinson's disease. A second type represents diseases that have a substantial genetic component, but are multifactorial and are greatly influenced by the environment. Examples of these are diabetes, lymphoma, atherosclerosis, alcoholic cirrhosis and anxiety disorders. Nonhuman primates are also ideally suited to the role of animal models in the new area of human gene therapy. In the future, biomedical research will focus increasingly on genetic manipulations such as the transfer of genes from one individual to another to correct genetic diseases, particularly those diseases caused by single recessive gene defects. Before gene transfers are attempted in humans, they should be done in nonhuman primates. In a real sense, nonhuman primates, as animal models, represent the "step to man."