Margaret Breit Thompson, 79, a former health educator of the American Cancer Society who was active in Alexandria churches, died of congestive heart failure June 29 at Inova Alexandria Hospital.

Mrs. Thompson, who lived in Alexandria, worked for the American Cancer Society from 1964 to 1972, lecturing at high schools and libraries about cancer and the dangers of smoking.

She was a deacon and elder at Mount Vernon Presbyterian Church and sang in its choir as well as in the choirs at St. Aidan's Episcopal Church and Church of the Resurrection Episcopal, both in Alexandria.

In 1990, Mrs. Thompson served as a lay chaplain at Mount Vernon Hospital.

As a young woman in her native Missouri, she graduated from St. Joseph's Junior College and Barnes Hospital school of nursing, and she received a master's degree in public health nursing from Washington University in St. Louis.

In 1946, she and her husband, James Neal Thompson, settled in the Washington area, and in those early years, she devoted her time to raising a family. She also was active in the Cub Scouts and Girl Scouts.

Her husband died in 1976. Survivors include three children, Mary K. Pruiett of Somers, N.Y., James N. Thompson Jr. of Arlington and Margaret J. Hays of Richmond; a sister; and four grandchildren.