Dr. Irwin Freitag, 57, a Bethesda dentist for 31 years, died of cancer Thursday at his home there.

Dr. Freitag, who specialized in prosthodontics, the branch of dentistry dealing with replacement of missing teeth, also was an instructor in the department of oral surgery at Georgetown University's School of Dentistry, from which he graduated in 1944.

He was a past president of the Southern Maryland Dental Society and the Bethesda Reciprocityu Club. His professional memberships included the Maryland State Dental Association, the American Prosthodontic Society. He also was an honorary member of the Sociedale Brasileira de Reabilitacao Oral.

Dr. Freitag was born in Brooklyn, N.Y., and earned a bachelor's degree from Muhlenberg College in Allentown, Pa. He did postgraduate work at Georgetown University's School of Dentistry, the University of Pennsylvania and the Hartford (Conn.,) Dental Society.

During World War II he served as a captain in the Army Dental Corps in the Pacific theater.

Dr. Freitag attended the Church of the Little Flower in Bethesda and was a member of the Men of St. John's and a trustee of the Catholic Youth Organization.

Survivors include his wife, M. Bernardien, a son, William, and a daughter, Sara, all of the home in Bethesda; four other sons, Joseph H., of Richmond, Michael, of Columbia, Md., David, of Washington, and Richard, of Alexandria; another daughter, Janet Albright, of Baton Rouge, La.; his mother, Rose Freitag, of Brooklyn; an sister Salene Frishberg, of Israel, and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to St. John's College High School, or to the Vincent T. Lombardi Cancer Research Center at Georgetown University Hospital.