Endometriosis is probably a complex trait, like diabetes, hypertension or asthma, which implies that the disease is caused by an interaction between multiple genes and the environment. This paper reviews the evidence, in both human and nonhuman primates, that supports the theory of a genetic basis to endometriosis. The OXEGENE study, which aims to identify susceptibility genes in endometriosis through linkage analysis, is also described. DNA is being collected from families containing at least two sisters with surgically confirmed revised American Fertility Society stage III and IV disease to carry out a genome-wide search for susceptibility genes. Candidate genes involved in galactose metabolism and dioxin detoxification are also being analyzed. The identification of genes conferring susceptibility to endometriosis may lead to a better understanding of disease etiology and, in time, improved therapeutic strategies and diagnostic methods.