There has been the biggest annual rise since 1990 in the proportion of GCSE exam entries awarded the best grades.
This year 65.7% of the exams taken were awarded A* to C grades, a rise of 2.4 percentage points from 63.3% last year, the exam boards reported.
As hundreds of thousands of teenagers in England, Wales and Northern Ireland received their results, the pass rate rose again, to 98.4%.
More than a fifth - 20.7% - were awarded either A* or A grades.
The number of entries fell to a five-year low of 5.7 million entries this summer compared with 5.8 million in 2007, down 2.7%.
GCSE RESULTS SUMMER 2008
Overall pass rate 98.4%
Overall A*-C grades: 65.7%
Northern Ireland A*-C: 74.5%
England A*-C: 65.5%
Wales A*-C: 65.0%
The provisional statistics were published by the Joint Council for Qualifications (JCQ), which said the decline was bigger than would be expected from the drop in the number of 16-year-olds in the population.
The director general of the AQA exam board, Dr Mike Cresswell, said: "Some young people are focusing their efforts on fewer GCSEs.
"That has been a trend that has been happening since 2003."
He said the average number of GCSEs taken per student had fallen from just over eight in 2003 to just under eight this year.
Another factor was that more students had taken English and maths exams early, in November, and these were not included in these summer results.
National and regional variations
As usual entries from Northern Ireland outperformed the rest. There, 26.4% were awarded A* or A grades and 74.5% A* to C.
In England, 20.6% achieved A*/A and 65.5% A* to C. In Wales the figures were 18.9% and 65%.
Scotland's exam results were released two weeks ago.
Luke, Ryan and Becky open their results
A Welsh Assembly Government spokesperson said their results compared very favourably with England's.
"Our overall pass rates are identical, the increase in pass rates is identical and the difference in proportion of A*-C grades is less than one percentage point - these figures do not constitute a gap in performance let alone a widening one.
"We have different assessment policies to those in place in England, this is true, but the results published today prove that Wales makes positive and consistent progress without the sometimes divisive influence of onerous testing regimes, league tables and tightly defined targets."
It will be another couple of months before statistics are available on how well students have done, such as what percentage achieved five or more good grades.
In England, confirmed results for each school will be published in the annual "league tables" in January as usual, though some may publicise their own results. Other parts of the UK do not publish such tables.
This year for the first time the JCQ has published a regional breakdown of results within England, which accounts for the bulk of the exam entries.
This showed that the improvement in A grades between 2002 and 2008 was greatest in London - from 18% to 23.4% (up 5.4 percentage points) - and the North East, from 11.7% to 17% (up 5.3).
London's improvement is telling because schools there have been put under the spotlight in a "challenge" programme now being applied nationally.
English education ministers want at least 30% of pupils in each school to attain five A*-C grades, including maths and English, by 2011.
They regard 638 schools as under-performing and have made them subject to their "National Challenge", with extra funding to improve their results.
Tory education spokesperson Maria Miller says too many students leave without 'good' GCSEs
The general secretary of the Association of School and College Leaders (ASCL), Dr John Dunford, says the programme has put extra pressure on schools.
He also warned against judging schools on one year's results.
"The GCSE results are most significant for the pupils themselves," he said.
"We have got to be reasonable because it takes time to turn schools around."
Congratulating students on their results, Schools Minister Andrew Adonis said: "Our message to young people is to stay on in education or training, whatever your results, and go on to fulfil your full potential."
he said almost half of all young people who stayed on would qualify for the Education Maintenance Allowance, a weekly payment of up to £30.
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