However, Welsh youngsters still marginally outperform their English counterparts, whose results also stabilised, with a 97.9 per cent pass (grades A* to G).
More Welsh students got higher-grade passes this year, with 59.8 per cent of entries resulting in A* to C grades, compared 0.6 percentage points up on 1999. Just over one in 20 got a top A* grade, a slight improvement.
Rosemary Butler, Welsh assembly education secretary, said: "We should all feel proud that today's results represent our best-ever performance.
"These results are a further step toward achieving the demanding targets that have been set for Wales."
Brian Evans, examinations secretary at the Welsh Joint Education Committee, said it was difficult to say if results had levelled out. "There are lots of strategies in place to continue improvements. We would have to see how that filters through the system."
Gareth Matthewson, treasurer of the Wesh National Association of Head Teachers and a Cardiff secondary head, said expecting improvements year on year was unrealistic. "Results have been maintained from last year, and those were much improved on previous years. It takes a while for improvement strategies to work through, and there is a limit.
"I'm sure in the medium to long-term, we will see more improvements because of the hard work of teachers, both primary and secondary," he said.
The biggest subject improvements were in geography, with A*-to-C passes up nearly 4 percentage points to 61 per cent, and Welsh (as a first language) - up 3 points to 69.7.
There were also significant improvements in English, French, biology and chemistry.
However, Welsh (as a second language) passes were down 2 percentage points, with single science awards at the higher grades down 5 points.
Subject entries in Wales were up 2.6 per cent on last year, to 227,993. Following a pilot last year, pupils will be able to ask to see their marked scripts in GCSE English, maths and Welsh.