All Might was gravely injured in a past battle with the villain All For One, where he took a massive blow to his side/abdomen that destroyed much of his respiratory system and stomach, leaving him able to use his power only in short bursts and giving him his skinny, emaciated form.

Quick Scoop: What Happened To All Might?

In the My Hero Academia story, All Might’s iconic injury comes from an earlier, off‑screen clash with his arch‑enemy All For One, years before the series begins. During that fight, All For One strikes All Might in the left side of his torso, damaging vital internal organs and nearly killing him.

Because of that injury, All Might loses a large part of his stomach and significant portions of his respiratory system, which is why he:

  • Can only maintain his muscular hero form for limited periods.
  • Frequently coughs up blood when he overexerts himself.
  • Appears extremely thin and frail in his “true” form.

How Did The Injury Change Him?

The damage is described as so severe that even advanced healing quirks in the series cannot fully repair it, leaving permanent limitations on his body. As a result:

  • Every time he pushes One For All too far, the stress aggravates his old wound.
  • His time as the active “Symbol of Peace” is shortened, forcing him to seek a successor (eventually choosing Midoriya).

Fans often discuss whether quirks like Recovery Girl’s or later Eri’s “Rewind” could have fixed him, but in-universe explanations emphasize that the injury was catastrophic, long-standing, and essentially beyond normal repair.

TL;DR: All Might got injured in a brutal past battle with All For One, who smashed his abdomen and wrecked his organs, leaving him with a permanent, life‑shortening wound that limits how long he can use One For All.

Information gathered from public forums or data available on the internet and portrayed here.