What illness caused the death of Holden's brother Allie?

Holden's brother Allie died of leukemia.

Allie Caulfield is Holden's younger brother in The Catcher in the Rye, and his death from leukemia is one of the defining tragedies of Holden's life. Allie died on July 18, 1946, when he was eleven years old and Holden was thirteen. Holden remembers him as intelligent, kind, and unusually good-hearted, which makes the loss feel even more devastating.

A central symbol of Allie's memory is his left-handed baseball mitt. Allie had written poems in green ink on the glove, and Holden treasures it because it preserves something personal and tangible about his brother. Holden's composition for Stradlater focuses on that glove, but Stradlater's dismissive reaction makes Holden feel that Allie's memory has been insulted.

Allie's death also helps explain Holden's grief, anger, survivor's guilt, and fixation on protecting children from growing up. The novel does not present influenza, tuberculosis, or pneumonia as the cause. Leukemia is a cancer of blood-forming tissues, but the book uses the illness mainly as part of Holden's emotional history rather than as a medical case study.