Mrs. Onassis, who had enjoyed robust good health nearly all her life, began being treated for non-Hodgkin's lymphoma in early January and had been undergoing chemotherapy and other treatments in recent months while continuing her work as a book editor and her social, family and other personal routines.

But the disease, which attacks the lymph nodes, an important component of  the body's immune system, grew progressively worse. Mrs. Onassis entered the New York Hospital-Cornell Medical Center for the last time on Monday but returned to her Fifth Avenue apartment on Wednesday after her doctors said there was no more they could do."  http://www.arlingtoncemetery.net/jbk.htm

A family member reports that she had leukemia, AML 

The wife of John Fitzgerald Kennedy, she married Greek financier Onassis several years after JFK's death in 1963. She became a widow the a second time when Onassis died and was again single when she died of cancer in 1994. Due to this status, she was laid to rest next to JFK and their two infant children in Arlington National Cemetery:  A prematurely born son, Patrick Bouvier. Four months later, they buried his father, the nation's president. A stillborn daughter was brought to lie in what would become a family cemetery for the Kennedys.