At the time of the explosion Sadako was at home, about one mile from Ground Zero. By November 1954, chicken pox had developed on her neck and behind her ears. Then in January 1955, purple spots had started to form on her legs. Subsequently, she was diagnosed with leukemia, which her mother referred to as "an atom bomb disease." 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 and folded it into a paper crane, in reference to the Japanese story that one who folds 1,000 cranes will be granted a wish. A popular version of the story is that she 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. This comes from the book Sadako and the Thousand Paper Cranes. An exhibit which appeared in the Hiroshima Peace Memorial Museum stated that by the end of August, 1955, Sadako had achieved her goal and continued to fold more cranes.
Though she had plenty of free time during her days in the hospital to fold the cranes, she lacked paper. She would use medicine wrappings and whatever else she could scrounge up. This included going to other patients' rooms to ask to use the paper from their get-well presents. Chizuko would bring paper from school for Sadako to use.
During her time in the hospital her condition progressively worsened. Around mid-October her left leg became swollen and turned purple. After her family urged her to eat something, Sadako requested tea on rice and remarked "It's good." Those were her last words. With her family around her, Sadako died on the morning of October 25, 1955 at the age of 12.
After her death, Sadako's friends and schoolmates published a collection of letters in order to raise funds to build a memorial to her and all of the children who had died from the effects of the atomic bomb. In 1958, a statue of Sadako holding a golden crane was unveiled in the Hiroshima Peace Memorial, also called the Genbaku Dome. At the foot of the statue is a plaque that reads:
"This is our cry. This is our prayer. Peace on Earth."
There is also a statue of her in the Seattle Peace Park. Sadako has become a leading symbol of the impact of nuclear war. Sadako is also a heroine for many girls in Japan. Her story is told in some Japanese schools on the anniversary of the Hiroshima bombing. Dedicated to her, people all over Japan celebrate August 6 as the annual peace day.