Hair grows from its follicle at an average rate of a 1/2 inch per month. Each hair grows for 2 to 6 years, then rests, and then falls out. A new hair soon begins growing in its place. At any one time, about 85% of the hair is growing and 15% is resting.

Baldness occurs when there is a progressive miniaturization of the follicle, resulting in shorter and finer hair. The end result is a miniature follicle with no hair. The cause of the failure to grow a new hair is not well understood, but it is associated with a genetic predisposition and the presence of endocrine hormones (androgens, the male sex hormones), especially dihydrotestosterone, which is converted from testosterone. The follicles, however, remain alive, suggesting the possibility of new growth.

The typical pattern of male pattern baldness begins at the hairline. The hairline gradually recedes to form an "M" shape. The existing hair may be finer and does not grow as long as it did formerly. The hair at the crown also begins to thin, and eventually the top points of the hairline "M" meet the thinned crown, leaving a horseshoe pattern of hair around the sides of the head.

Hair loss in patches, diffuse shedding of hair, breaking of hair shafts, or hair loss associated with redness, scaling, pain, or rapid progression could be caused by other conditions.