Charles S. Dewey Sr., 100, a principal organizer of the Washington Hospital Center and a former banker, government official and congressman, died of pneumonia Thursday at his home in Washington.

In the course of his career, Mr. Dewey was in the real estate business in Chicago, served in the Navy in World War I, was a banker in Chicago in the early 1920s, assistant secretary of the Treasury for fiscal affairs from 1924 to 1927, financial adviser to the Polish government from 1927 to 1931, and then a banker in Chicago again until 1941.

In that year, he became a Republican congressman from Illinois and remained in the House of Representatives until 1945. After leaving Congress, he was a vice president of the Chase National Bank. In 1948, he was appointed agent general of the Joint Congressional Committee on Foreign Economic Cooperation. His job was to act as a congressional watchdog over the Marshall Plan, the foreign aid program designed to restore the economy of Western Europe after World War II. He held that job for three years.

His other activities included serving as national treasurer of the American Red Cross in 1927 and as chairman of the D.C. Chapter of the Red Cross from 1957 to 1961. He also was a member of the national board of the Boy Scouts of America.

Mr. Dewey had made his permanent residence in Washington since 1941. He was active in numerous civic enterprises here and the most memorable of his services, perhaps, was the work he did for the Washington Hospital Center.

The hospital center combined the staffs of three existing hospitals -- Garfield, Emergency and Episcopal -- in a new physical plant. It was built with federal assistance at a cost of about $25 million and opened in 1957.

Mr. Dewey first became interested in the project in 1946, when he was named to the board of directors of Emergency Hospital. In 1951, when he finished his work for the Marshall Plan, he was elected president of the board of Garfield Hospital. When he was named president of the nonprofit corporation that was pushing the hospital center project, he resigned his post at Garfield to devote full time to the enterprise. He was credited with ensuring congessional support for the project, raising funds and acting as an effective catalyst among the three existing hospitals.

Mr. Dewey was the first president of the Washington Hospital Center and remained in that job until 1959. His work on behalf of the Red Cross and other groups continued for many years after that.

Mr. Dewey was born in Cadiz, Ohio, and grew up in Chicago. He graduated from Yale University in 1904 and then returned to Chicago, where he was in the real estate business with his father. After his World War I service, during which he was stationed on the battleship Mississippi, he returned to Chicago and became a vice president of the Northern Trust Co. There followed his work at the Treasury Department and in Poland, the government of which made him an honorary director of the Bank of Poland for life. During the 1930s, he returned to banking in Chicago.

Mr. Dewey also spent much time in France during his life and owned property in Brittany. He received decorations from Poland, France, Romania and Yugoslavia.

He was a member of the Sons of the American Revolution, the Society of the Cincinnati, the Chicago Club and the Metropolitan Club.

His first wife, Suzette de Marigny Mall Dewey, died in 1957. His second wife, Elizabeth Zolnay Smith Dewey, died in 1977. A daughter, Mrs. Frederick M. Alger, and two sons, Charles S. Jr. and Army Lt. Col. A. Peter, also died. Col. Dewey was killed in action while serving with the Office of Strategic Services in Saigon, Vietnam, in 1945.

Survivors include one daughter, Mrs. Edward Byron Smith of Chicago, 10 grandchildren, 18 great-grandchildren and one great-great-grandchild.