先天性遗传多毛症（hypertrichosis）是一种返祖现象，患者一般在出生后不久全身就长满了乌黑而坚硬的毛，有如传说故事中的狼人，因此又称狼人综合症，但身体其他状况与常人无异，智力发育也正常，经常被称为“毛孩”。

19世纪意大利有一位著名的“胡子小姐”朱莉娅·帕斯特罗娜就是先天性遗传多毛症患者。她出生于1834年，身高只有1.38米，浑身除手掌和脚掌之外都长有坚硬而浓密的毛。朱莉娅后来被父亲抛弃，被一位流浪艺人收留。随后二人结为夫妻，朱莉娅还产下一个女婴，这个女婴同样浑身长满了毛，只存活了36个小时就夭折。据称朱莉娅的遗体目前保存在挪威的奥斯陆医学研究所。

Hypertrichosis (also called Ambras syndrome) is an abnormal amount of hair growth over the body; extensive cases of hypertrichosis have informally been called werewolf syndrome, because the appearance is similar to the mythical werewolf. The two distinct types of hypertrichosis are generalized hypertrichosis, which occurs over the entire body, and localized hypertrichosis, which is restricted to a certain area. Hypertrichosis can be either congenital (present at birth) or acquired later in life. The excess growth of hair occurs in areas of the skin with the exception of androgen-dependent hair of the pubic area, face, and axillary regions.