What disease caused Allie Caulfield's death?

What disease caused Allie Caulfield’s death? The answer is leukemia.

Allie, Holden Caulfield’s younger brother, died of leukemia before the main events of the novel. Holden remembers him as unusually kind and intelligent, and he preserves that memory through Allie’s treasured baseball mitt, on which Allie wrote poems in green ink.

The loss is one of the deepest sources of Holden’s grief and emotional instability. In a flashback, Holden recalls smashing the windows of the garage with his bare hands after learning of Allie’s death. That episode helps explain both his continuing guilt and his intense desire to protect children from the corruption he associates with adulthood.

Readers sometimes confuse Allie’s illness with the novel’s broader themes of depression and trauma, but those describe Holden’s emotional condition rather than the disease that killed his brother. The answer is a specific medical fact in the story: leukemia.