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Polycystic ovary syndrome (PCOS) is the most common endocrinopathy in women of reproductive age. Familial clustering of cases suggests that genetic factors play an important part in its aetiology. A number of studies of families with several cases of PCOS have produced results suggesting an autosomal dominant trait. Detailed analysis of a large number of affected families has, however, cast some doubt about the mode of inheritance. An autosomal dominant trait remains possible but a more complex aetiology seems more likely. The results of our recent studies support the concept of an oligogenic disorder in which genes affecting metabolic pathways in glucose homeostasis and steroid biosynthesis are both involved. We review evidence for an important role for the insulin gene minisatellite in the aetiology of anovulatory PCOS and for the gene coding for P450 cholesterol side chain cleavage (CYP11a) in the mechanism of excessive androgen secretion in women with polycystic ovaries. We propose that the heterogeneity of clinical and biochemical features in PCOS can be explained by the interaction of a small number of key genes with environmental, particularly nutritional, factors.