In Memento, what condition causes Leonard Shelby’s memory loss?

In Memento, Leonard Shelby’s memory loss is caused by anterograde amnesia.

Anterograde amnesia prevents a person from forming new long-term memories after a particular event. In Memento, Leonard can remember much of his life before the attack on him and his wife, but he cannot reliably retain new experiences. The film turns this medical premise into its central storytelling device.

Leonard uses Polaroid photographs, handwritten notes and tattoos to preserve information he would otherwise lose. These aids are practical within the story, but they do not guarantee that his interpretations are correct. The film therefore explores the difference between recorded evidence and objective truth.

Retrograde amnesia, by contrast, involves losing memories from before an injury or event. Memento combines a reverse-running storyline with a parallel black-and-white storyline, making the audience share Leonard’s uncertainty without suggesting that the editing itself is a medical symptom.