SUMMARY Patients with idiopathic hirsutism form a heterogeneous group with a number of hormonal abnormalities, those most consistently demonstrated being an increase in free plasma testosterone levels, an increase in testosterone production rate and a decrease in sex hormone binding globulin levels. In most patients the principal source of androgen excess appears to he ovarian. Some have additional adrenal hyperandrogenism but a pure adrenal source is probably rare. Idiopathic hirsutism may be regarded as part of a clinical spectrum of ovarian hyperandrogenism. The ovarian patholoqy present in the majority of cases is probably that of polycystic ovarian disease. This may be a genetically determined disorder with a modified dominant form of inheritance. Ethnic and familial variations in amount and type of hair growth in androgen dependent sites undoubtedly do occur. Nevertheless the finding of normal plasma androgens in many women labelled ethnically or constitutionally hirsute may be the result of inappropriate blood sampling and failure to measure free plasma testosterone or testosterone production rate.