Male pattern baldness (also called androgenetic alopecia) is a progressive loss of scalp hair in men that begins in the twenties or early thirties. About 95 percent of all cases of hair loss are the result of male pattern baldness. Androgenetic alopecia occurs much more frequently in men than in women. It affects roughly 35 million men in the United States starting any time after puberty when blood levels of androgens rise.

While some types of hair loss are easily reversible, male pattern baldness is more permanent. It occurs in a characteristic pattern on the scalp: hair loss usually begins at the temples and at the top of the head toward the back, causing a receding hairline and a bald spot. Hair loss may continue until the two sections become joined, leaving a horseshoe-shaped area of hair on the sides and back of the head. Balding may begin at any age after puberty, even in the middle teens, and can range from partial loss to complete baldness. Male pattern baldness progresses slowly and is not associated with redness, itching, or pain. Currently, there is no way to prevent male pattern baldness from occurring.

Whatever the exact causes of male pattern baldness may be, it is a hereditary trait. There are multiple genetic factors that influence male pattern baldness. A tendency toward baldness in the men on either the mother's or father's side of a man's family indicates a genetic predisposition to baldness. The speed, pattern, time of onset and degree of balding are all influenced by heredity. Generally, the earlier the onset of balding, the more extensive the degree of hair loss will eventually be.