In November 1954, Sadako developed swellings on her neck and behind her ears. In January 1955, purple spots had formed on her legs. Subsequently, she was diagnosed with leukaemia. She was hospitalized on February 21, 1955, and given, at the most, a year to live.
On August 3, 1955, Sadako's best friend Chizuko Hamamoto came to the hospital to visit and cut a golden piece of paper into a square to fold it into a paper crane, in reference to the ancient Japanese story that promises that anyone who folds a thousand origami cranes will be granted a wish by a crane. A popular version of the story is that Sadako fell short of her goal of folding 1,000 cranes, having folded only 644 before her death, and that her friends completed the 1,000 and buried them all with her.
With her family around her, Sadako died on the morning of October 25, 1955 at the age of 12.
After Sadako died, her friends and her schoolmates published letters so that they could raise the funds in order to build Sadako a memorial to her and the other children in Japan who had died due to the effects of the atom bomb. In 1958, the monument was complete. It was a statue of Sadako, and in her hands is a golden crane. This monument is located at Hiroshima Peace Memorial, otherwise known as Genbaku Dome. The plaque that is on the memorial states, "This is our cry. This is our prayer. Peace in the world."
The origami crane has now become an international symbol of peace.