The number of top GCSE results in Northern Ireland dropped by 2.2 percentage points following a shake-up of the grading system.
This was the first year a new A* to G grading system was used for all locally-awarded qualifications to bring them into line with that in England.
Michael Gove first introduced the significant change in a bid to drive up standards.
Northern Ireland's A* issued by the local awarding body has this year been fully realigned to the 9 grade in England. A new C* has also been introduced.
A* grades dropped by 2.2 percentage points from 9.9% last year to 7.7%.
The proportion of candidates awarded A*-C increased by 1.1 percentage points from 81.1% last year to 80.7%.
Overall, students performed well, with slight increases at grade C and above.
Boys narrowed the performance gap with girls to 7.1 percentage points.
GCSE maths saw a 3.2 percentage point increase at A*-C from 68.1% to 71.3%.
In 2016 former DUP education minister Peter Weir decided that Northern Ireland should realign to the new English grading system.
That overturned a decision not to do so a year earlier by his predecessor as minister, John O'Dowd, which led the two largest English GCSE exam boards to say they would not offer GCSE courses in Northern Ireland..
In Northern Ireland this year, 30.5% of boys and girls received an A. That compares with 20.8% across the UK as a whole.
A quarter of boys and 35.7% of girls obtained an A.
In English there was a half a percentage point increase at grade C and above, and maths saw a 3.3 percentage point increase.
More pupils were studying subjects like health and social care, construction and drama, and fewer students took German, ICT and Spanish.
Science single award moved out of the most popular subjects for boys and was replaced by geography.
Home economics and ICT was replaced by history and single award science among girls.
A 1.4 percentage point decrease in proportional entry for STEM subjects (science, technology, engineering and mathematics) was noted but it was too early to discern a trend.
There was a 0.3 percentage point decrease in proportional entries for languages.
This year saw a 5.2% decline in the number of entries. The trend was particularly marked among 15 and 17-year-olds.