William Ludlow Ritchie, 78, a retired brigadier general in the Air Force who served as an aide at several of the major conferences of allied leaders during World War II, died Sunday at the Cape Cod Hospital in Hyannis, Mass., following surgery for an aneurysm.

Gen. Ritchie, who had maintained a home in Washington since 1935, was vactioning at his summer house on Nantucket Island when he was stricken.

He was born in Helena, Ark., graduated from the U.S. Military Academy at West Point in 1925, and entered the old Army Air Corps in 1929.

In 1935, he was assigned to Washington and became an aide and pilot for Harry H. Woodring, the secretary of war in the second administration of President Franklin D. Roosevelt. He also became a White House aide.

After the United States entered World War II, he was chief of plans of the Southwest Pacific Theater section at the War Department. He later served as a liaison officer between Gen. George C. Marshall, the Army chief of staff, and Gen. Douglas MacArthur, the commander in the Southwest Pacific.

Gen. Ritchie attended the allied conferences at Quebec, Cairo, Teheran, Yalta and Potsdam. It was at these meetings that Roosevelt, Prime Minister Winston Churchill and Generalissimo Joseph Stalin and other leaders settled major policy questions for the war. President Harry S. Truman represented the United States at the Potsdam Conference shortly after the end of the war.

By this time, Gen. Ritchie had served as chief of the air staff of the U.S. Mission to the Soviet Union. He also was a military attache in Moscow during those years. After the war, he helped organize the Air Force as a separate service.

He retired from the Air Force in 1951 for reasons of health. During the 1950s, he was a business consultant in Washington and then retired a second time.

Beginning in 1952, Gen. Ritchie became active in the English Speaking Union. He was president of its Washington branch in 1961 and was a national vice president in 1967. He was a member of the national board of the organization at the time of his death.

Gen. Ritchie's military decorations included the Distinguished Service Medal, the Legion of Merit and the Air Medal.

For his services to the English Speaking Union, Queen Elizabeth II of Britain made him an Honorary Commander in the Civil Division of the Order of the British Empire.

Gen. Ritchie was a member of the Army-Navy Club, the Chevy Chase Club, the Metropolitan Club and the Sankaty Golf and Beech Club in Nantucket.

Survivors include his wife, the former Eleanor Preston, of Washington; a son, William Ludlow Jr., also of Washington; a daughter, Louise Beale, of Evergreen, Colo. and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to St. John's Episcopal Church, Lafayette Square, Washington, D.C. or to the Air Force Aid Society, 1117 N. 19th St., Arlington, Va., 22209.