A study of five subjects - accountancy and finance, English, French, geography and modern studies - in 1985, 1992 and 1998 shows levels of demand broadly comparable and pass rates steady.
The SQA initiated the project following an earlier investigation by the Scottish Council for Research in Education which also concluded that the standard of four Highers between 1987 and 1994 had not changed over 10 years.
New SQA scrutiny groups comprised two experienced teachers, a higher education representative and a SCRE researcher acting as an independent observer.
An SQA spokesman said: "Comparing work from different years did present difficulties, not only because each year set different assessment tasks and sampled diferent aspects of the range of content, but also because parts of the examination had been changed or discontinued. Also, information about the original marking and grading of scripts was incomplete.
"Within these constraints, though, the groups concluded that the level of 1985 and 1992 candidates was broadly comparable to 1998 candidates."
Levels of demand, defined by syllabuses and testing, were also broadly comparable.
David Eaglesham, general secretary of the Scottish Secondary Teachers' Association, said: "The reality is that the system is being refined while standards are being maintained. What this report shows is that pupils continue to work hard and that teachers are more focused in teaching to the requirements of SQA exams."
Annual monitoring of specific subjects is ongoing. Science and technology is being examined this year and again in two years.