Objective: To determine the prevalence of idiopathic hirsutism among a population of consecutive hirsute patients. Design: Prospective cohort study. Setting: University-based clinic. Patient(s): Premenopausal women with a complaint of hirsutism who were not receiving hormonal therapy. Intervention(s): Evaluations for total and free testosterone, (T), 17- hydroxyprogesterone (17-HP), and DHEAS serum levels. Main Outcome Measure(s): Ovulatory function in women with cycles of ≤35 days in length was assessed with a basal body temperature (BBT) calendar and day 22-24 progesterone levels. Result(s): Of 132 consecutive hirsute women studied, 68 had cycles of >35 days in length. Of the remaining 64 patients, 25 also had oligo/anovulation by BBT and day 22-24 progesterone level. Of the 39 patients with hirsutism and regular ovulatory function, 22 had total and free T and DHEAS levels within normal limits. Conclusion(s): If idiopathic hirsutism is defined by the presence of hirsutism, regular ovulation, and normal androgen levels, only 17% of consecutive hirsute patients can be diagnosed with the disorder. Alternatively, if idiopathic hirsutism is based solely on the presence of hirsutism and regular ovulation, regardless of androgen levels, then 29% of the total hirsute population can be considered as having idiopathic hirsutism. Importantly, 40% of hirsute patients with a history of 'regular cycles' were actually oligo/anovulatory, indicating the need to objectively assess ovulatory function in such patients.