TOKYO -- Japan's Princess Kikuko, the emperor's aunt and an outspoken supporter of allowing women to assume the throne, died Saturday at age 92 at St. Luke's Hospital here, where she was undergoing treatment for kidney problems, the Imperial Household Agency said.
The royal family began observing a five-day mourning period on Saturday. A funeral was expected to be held later this month.
Princess Kikuko, also known as Princess Takamatsu, was the widow of the late Emperor Hirohito's younger brother Takamatsu and the granddaughter of Yoshinobu Tokugawa, Japan's last shogun, or feudal lord.
"The princess made major contributions to society in many areas, including medicine," Prime Minister Junichiro Koizumi said in a statement Saturday.
Princess Kikuko had been a champion of cancer research in Japan since the 1930s. Using money donated by the public, she established a cancer research fund in 1968, organizing symposiums, and awarding scientists for groundbreaking work.
She was seen as one of the most progressive members of Japan's tradition-bound royal family, the world's oldest hereditary monarchy.
She had no children. Her husband, a philanthropist and an adviser to Hirohito in the 1940s, died in 1987 of lung cancer.
Many Japanese were shocked by her 1995 decision to publish his diaries -- written before and during World War II and containing criticism of Japan's wartime military -- despite opposition from the Imperial Household Agency.
In 2002, after Crown Prince Naruhito and Princess Masako had a daughter, Princess Kikuko was the first royal to publicly call for changes to a postwar law that allows only male heirs to assume the Chrysanthemum Throne.
In an article she wrote for a women's magazine, she argued that having an empress was "not unnatural" because women had assumed the throne in the past, most recently in the 18th century.