Franklin Delano Roosevelt Jr., a former congressman and the third son of Franklin and Eleanor Roosevelt, died of cancer Wednesday, his 74th birthday, at the Vassar Brothers Hospital.

Roosevelt, who lived in nearby Millbrook, was admitted to the hospital on July 21, said Charles Gill, a spokesman. He died at 8 a.m.Christopher Roosevelt said his father died of lung cancer. "It was a very late diagnosis and very fast moving."

Roosevelt, a Liberal-Democrat who came of age while his father was in the White House, began his own political career in 1949 when he defeated the Tammany Hall-backed Democratic candidate for the 20th Congressional District seat. He served three terms in Congress, retiring in 1955.

Although Roosevelt bore a striking resemblance to his father and even shared the habit of throwing his head back when he laughed, he said in a 1954 interview with The Associated Press that his father did not "train him in politics."

He said he was away at boarding schools most of the time. "It's true the family was always holding discussions, but you can't say I learned politics at my father's knee."

But in a Saturday Evening Post article, young Roosevelt remembered his father urging him to become a lawyer because it was good preparation for a political career.

He said one of the most valuable lessons he learned from his father, who frequently visited him at law school, was "not worrying."

He recalled that his father "always did the best he could, and slept soundly at night. He never worried. Neither do I."