A obsessive-compulsive disorder afflicting somewhere between two and four million Americans which causes them to pull out their own hair. I am not making this up. Most of them are women who begin suffering in their teens or twenties. Prozac combats this quite well, and can sometimes make it disappear completely.

From the DSM IV:

312.39 Trichotillomania

Diagnostic Features
The essential feature of Trichotillomania is the recurrent pulling out of one's own hair that results in noticeable hair loss (Criterion A). Sites of hair pulling may include any region of the body in which hair may grow (including axillary, pubic, and perirectal regions), with the most common sites being the scalp, eyebrows, and eyelashes. Hair pulling may occur in brief episodes scattered throughout the day or in less frequent but more sustained periods that can continue for hours. Stressful circumstances frequently increase hair-pulling behavior, but increased hair pulling also occurs in states of relaxation and distraction (e.g., when reading a book or watching television). An increased sense of tension is present immediately before pulling out the hair (Criterion B). For some, tension does not necessarily precede the act but is associated with attempts to resist the urge. There is gratification, pleasure, or a sense of relief when pulling out the hair (Criterion C). Some individuals experience an "itchlike" sensation in the scalp that is eased by the act of pulling hair. The diagnosis is not given if the hair pulling is better accounted for by another mental disorder (e.g., in response to a delusion or a hallucination) or is due to a general medial condition (e.g., inflammation of the skin or other dermatological conditions) (Criterion D). The disturbance must cause significant distress or impairment in social, occupational, or other important areas of functioning (Criterion E).