Only Marilyn and JFK knew for sure, and the true nature of their relationship died with them. However, it does appear that Kennedy and Monroe did have an affair, and it has been corroborated in print by several people close to Kennedy. One of JFK's other purported mistresses, Judith Exner, claimed Monroe and Kennedy had an affair in her autobiography, Judith Exner: My Story (1977). Another source (Official and Confidential: The Secret Life of J. Edgar Hoover by Anthony Summers, 1993) claims that Marilyn wanted to marry Kennedy and that the President
had to break off their affair, [and] Monroe became even more depressed, and then turned to Robert Kennedy, who visited Monroe in Los Angeles the day that she died.
Author Richard Reeves claims that
"the logistics of Kennedy's liaisons... required secrecy and devotion rare in the annals of the energetic service demanded by successful politicians."
One of Marilyn's friends, Robert Slatzer, claims that Monroe discussed sleeping with Kennedy, and that
"She insisted that he made love like an adolescent..."
Former Los Angeles Mayor Sam Yorty was quoted as saying that the First Lady was already aware of the Monroe-Kennedy liaisons by April 1962.
"Jackie knew about the affair by then..."
The supposed Kennedy-Monroe affair is still a hot topic 50 years later, but one must decide for themselves whether it was real or simply a case of Washington/Hollywood gossip.