(FORTUNE Magazine) – In Memento, Guy Pearce's character drives a hot Jaguar, wears designer suits, and has sex with Carrie-Anne Moss. So why is he so angry? Well, ever since he was thumped on the head, he can't form new memories, which makes it pretty tough for him to track down his wife's killer. The premise makes for a nifty thriller, but surely that peculiar form of amnesia exists only on the silver screen. Right?

Wrong. The movie is based on a real condition--anterograde amnesia--caused by damage to the hippocampus, a part of the brain essential for creating new memories. "The film was incredibly realistic," says Donald Davidoff, a psychologist in charge of special care at McLean Hospital in Belmont, Mass., and a member of the Harvard Medical School faculty. "This kind of thing really does happen." (Davidoff once treated a patient who spent hours in an elevator as he tried to figure out where he was going and how long he had been there.)

Short-term memory loss is usually caused by Alzheimer's disease or a thiamine deficiency combined with alcoholism; a simple blow to the head isn't likely to cause long-standing anterograde amnesia. But Davidoff says it's "within the realm of possibility." Sex with Carrie-Anne Moss? That's another story.

--Jason Tanz