M*A*S*H was a massively successful show that ran from 1972 to 1983. It was a humorous (although sometimes serious) look at the personnel who worked at a Mobile Army Surgical Hospital (hence, MASH) during the Korean War.
The show became massively successful and by the time it ended in 1983, it was one of the most watched series of all time. In fact, the 135 minute finale, titled "Goodbye, Farewell and Amen," was watched by 121.6 million viewers. That was more than half of the US population.
The finale was the top watched television broadcast in American history from 1983 until 2010, when Super Bowl XLIV surpassed it in total viewership. However, M*A*S*H still holds the record for total share and ratings. In fact, given how fragmented our media is today, it's very possible that it will remain the highest rated broadcast in American history forever.