He died in a hospital after a short illness, his office in Geneva announced. No cause of death was given.
For nearly 40 years, Prince Sadruddin was closely associated with the work of the United Nations, specializing in disaster relief work.
After three years as deputy high commissioner for refugees, he became the high commissioner in 1965 at 33, the youngest person ever to hold the position. He went on to be the longest-serving, remaining in office until 1977.
He led UN efforts to deal with the 10 million refugees created by the breakup of Pakistan and Bangladesh in 1971. He helped find nations to take in the thousands of Vietnamese refugees fleeing their country in the 1970s, the Asian victims of Idi Amin's dictatorship in Uganda and the refugees created by the long Sudanese civil war.
After giving up the high commissioner's job, Sadruddin continued to work for the United Nations, chiefly in the human rights and relief fields.
From 1988 to 1990, he took charge of UN relief operations in Afghanistan, seeking to resettle Afghan refugees after the withdrawal of Soviet forces.
From 1990 to 1991 he was the UN's relief coordinator for Iraq and Kuwait after the occupation of Kuwait and the subsequent Gulf War.
Sadruddin was twice a serious candidate for the top UN job. In 1981, when Kurt Waldheim tried unsuccessfully for a third term as secretary-general, Sadruddin drew more positive votes in the Security Council than anyone else. But he was blocked when the Soviet Union, which thought him too pro-Western, cast a veto. He tried and failed again in 1991 after Javier Perez de Cuellar stepped down. Once again he was perceived as too pro-Western, and the job went to Boutros Boutros-Ghali of Egypt.
During the latter years of his life Sadruddin, who lived in the Chateau de Bellerive on the shores of Lake Geneva, turned his attention and his money to artistic and environmental causes.
He amassed a particularly fine collection of Islamic art, for which a new museum is being constructed in Toronto. Through two organizations he created, the Bellerive Foundation and Alp Action, Sadruddin also was actively engaged in environmental work, particularly the preservation of the Alps and the protection of rare birds.
Sadruddin Aga Khan was born in Paris on Jan. 17, 1933, younger son of the late Sir Sultan Mohammed Shah, the 48th hereditary Imam of the Ismaili Muslims and the third Aga Khan.
Sir Sultan Mohammed Shah died in 1957. In his will, he passed over both Sadruddin and his elder half-brother, Prince Aly Khan, and named his grandson Prince Karim, then a student, to be 49th Imam and the fourth Aga Khan, saying the Ismailis needed a younger leader.
After graduating from Harvard University in 1954, Sadruddin did three years of post-graduate research at the Center for Middle Eastern Studies at Harvard.
He then went to work for the United Nations Education, Scientific and Cultural Organization in Paris, where he organized a campaign to rescue the ancient Egyptian temples of Abu Simbel, Philae and Kalabsha and the Nubian Christian churches.
[Sadruddin was married for five years to a model, Nina Dyer, and their 1962 divorce was headline news, The Associated Press reported. But he quickly shed his playboy image and managed overall to avoid the gossip that dogged many members of the dynasty.
[He shunned the racehorses, fast cars and diamonds favored by his half brother Aly, who was briefly married to the actress Rita Hayworth and who was the father of Karim, the Aga Khan IV.]