Dr. Henry A. Hornthal, 78, an area physician for about 50 years who specialized in obstetrics and gynecology, died Monday at his home in Arlington after a heart attack.

He maintained a practice in Washington until 1978 and in Alexandria until the time of his death. He had been an instructor at George Washington University's medical school from 1926 to 1929.

Dr. Hornthal had been president of the Arlington County Medicial Society, and president of the Washington alumni association of the Medical College of Virginia. He had graduated from the Richmond school in 1924.

He also had belonged to the American and Southern medical assoications, the D.C. Medical Society, and the American Society of Abdominal Surgeons.

Dr. Hornthal was a native of Norfolk and served as a Navy flight surgeon during World War II. He interned at St. Elizabeths Hospital in Washington and did post-graduate work at the Mayor Clinic.

He was a Mason and belonged to the Scottish Rite Temple and the Royal Order of Jesters. He also belonged to the Order of the Eastern Star, the Sons of the American Revolution, and the Society of mayflower Descendants.

Dr. Hornthal's wife, the former Ellen Hall, died on Aug. 3, 1979.

His survivors include a daughter, Ann H. Merlo of Elkin, N.C., and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to Alexandria Hospital.