Hypertrichosis, also called werewolf syndrome, is a genetic abnormality causing ezcessive hair growth on the face and body. There are two distinct types of hypertrichosis: 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 it can manifest later in life.

In the 19th and early 20th centuries numerous sufferers of this condition enlisted in circuses and sideshows. Many of them worked as freaks and were promoted as being genuine werewolves or ahving animal ancestry.