Wales has fallen further behind most of the UK for the highest grades at GCSE, results show.
But despite the widening gap, the results still represent Wales' best GCSE figures.
More than half a million students in England, Wales and Northern Ireland are picking up their results.
The figures show 66.4% of pupils in Wales received A* to C grades, compared to 69.1% for all of the UK.
The UK figures are made up of England, Northern Ireland and Wales, with Scotland's examination system organised separately.
The figures show that the overall GCSE pass rate in Wales has improved again, with 98.7% of entries now earning an A* to G grade, up slightly on last year.
Entries earning the top A* grade fell slightly, although almost one in five are still awarded at least an A grade.
First Minister Carwyn Jones said the young people of Wales had performed excellently and should be proud.
Meanwhile, the gap between the performance of girls and boys has narrowed a little, although girls continue to outperform boys in general.
The percentage of entries from girls earning an A* was 7.1%, compared to 5% for boys.
The gap between Wales and the UK for the highest grades has been widening for the last four years.
In 2006, the gap was 0.1%, but that has now widened to 2.7%.
Exam board the WJEC said the differences appeared to be largely a result of the larger independent sector in England.
Derec Stockley, WJEC's director of examinations and assessment, said initial analysis of the results obtained in the state sector alone show negligible differences.
"If you look at comprehensives in Wales and compare that to comprehensives and academies in England, which are grouped together, then the results are nearly identical," he said.
Mr Stockley said more research was needed into the fact the gap was widening and that historically there had been no difference in the results between Wales and the whole of the UK.
He said: "Obviously we are concerned. But the positive thing to mention is that this year's results are consistent with last year's figures, with actually a year on year improvement."
Rex Philips, Wales organiser for the teaching union NASUWT, said: "Whilst is right that the best and the brightest of our pupils and their teachers should be congratulated for their outstanding achievements at GCSE this year, we should not lose sight of the fact that pupils who achieve a D, E or G may well have excelled themselves.
"Many of those pupils and their teachers will have worked extremely hard to gain these grades and their efforts and achievements are as much a reason to celebrate as those gaining higher grades."
He said the concentration on A to C grades went against the ethos of the GCSE system which was introduced to replace the GCE and CSE examinations.