OBJECTIVE Polycystic ovary syndrome is one of the most common endocrine disorders but its aetiology remains unknown. It is highly prevalent within families, suggesting a genetic basic for the syndrome, but the mode of inheritance is unclear. The purpose of this study was to determine the mode of inheritance of polycystic ovary syndrome, within the families of affected individuals, by classic segregation analysis.

DESIGN All first degree relatives of affected individuals were screened for the presence or absence of polycystic ovaries in post‐menarchal‐premenopausal women and early onset male pattern baldness (MPB) in the males. In extended pedigrees, assignment of affected status in postmenopausal women was made by consideration of the clinical history alone.

PATIENTS Fourteen women (probands), presenting with a variety of clinical symptoms, were identified sequentially as having polycystic ovaries (PCO) by ultrasound scan. They were examined in detail to determine their family structure, clinical and endocrine status. Ten families were found to have sufficient members for further study.

MEASUREMENTS All family members had their body mass index calculated, their degree of hirsutism assessed using the Ferriman and Gallwey score and serum levels of gonadotrophins (FSH and LH), testosterone, prolactin and 17α‐hydroxyprogesterone measured by radioimmunoassay. A careful reproductive history was taken for each woman and any menstrual disturbance was noted. Obese probands had their glucose and insulin response to a standard 75‐g oral glucose tolerance test determined. Each male family member was also assessed for the degree and time of onset of balding.

RESULTS First degree female relatives of affected individuals had a 51 % chance of being affected. Early onset male pattern baldness (MPB) was found to be an accurate phenotype for obligate male carriers. Each family showed autosomal dominant inheritance for PCO with greater than 90% penetrance.

CONCLUSIONS We postulate that PCO and male pattern baldness are caused by alleles of the same gene which affect androgen production or action. The different frequencies of PCO and male pattern baldness arise from differing thresholds for phenotypic expression in females and males respectively. The modifying effects of other genes is the most likely explanation of the somewhat variable phenotype.