The Glasgow Coma Scale (GCS), used globally to measure a person's state of consciousness, was originally devised by a team of doctors at the Southern General Hospital.
Its anniversary was marked with a celebration at the National Piping Centre in Glasgow with more than 100 NHS Greater Glasgow and Clyde staff, past patients and students.
The simple measurement looks at visual, verbal and motor responses and ranks patients' responses. The GCS is used in 80 countries and has been translated into 60 languages.
Chairman of NHS Greater Glasgow and Clyde Andrew Robertson said: "The fact that it is being used throughout the world 40 years on is testament to the exceptional work and research which went into it in 1974."
Professor Sir Graham Teasdale, of Kilmacolm, recalled working on its development with his colleague Professor Bryan Jennett. He said: "It was a time when there was a lot going on with treating coma patients. There were dozens of different systems. We had a lot of problems with vague terms. They needed an international language.
"We were in the right place at the right time."