Hirsutism, the presence of terminal (coarse) hairs in females in a male-like pattern, affects between 5% and 10% of women. Of the sex steroids, androgens are the most important in determining the type and distribution of hairs over the human body. Under the influence of androgens hair follicles that are producing vellus-type hairs can be stimulated to begin producing terminal hairs (i.e., terminalized). The activity of local 5α-reductase (5α-RA) determines to a great extent the production of dihydrotestosterone (DHT), and consequently the effect of androgens on hair follicles. While there are two distinct 5α-RA isoenzymes, type 1 and type 2, the activity of these in the facial or abdominal skin of hirsute women remains to be determined. Although the definition of idiopathic hirsutism (IH) has been an evolving process, the diagnosis of IH should be applied only to hirsute patients with normal ovulatory function and circulating androgen levels. A history of regular menses is not sufficient to exclude ovulatory dysfunction, since up to 40% of eumenorrheic hirsute women are anovulatory. The diagnosis of IH, when strictly defined, will include less than 20% of all hirsute women. The pathophysiology of IH is presumed to be a primary increase in skin 5α-RA activity, probably of both isoenzyme types, and possibly an alteration in androgen receptor function. Therapeutically, these patients respond to antiandrogen or 5α-RA inhibitor therapy. Pharmacological suppression of ovarian or adrenal androgen secretion may be of additional, albeit limited, benefit. New therapeutic strategies such as laser epilation or the use of new biological response modifiers may play an important role in offering a more effective means of treatment to remove unwanted hair. Further investigations into the genetic, molecular, and metabolic aspects of this disorder, including only well defined patients, are needed.