Higher grades are disproportionately in London and the South East
Wide regional variations are exposed for the first time in a breakdown of GCSE exam results within England.
Exam boards have published statistics on the improvements in grades seen in recent years - with London and the North East leading the way.
Pupils in the North East displayed the biggest gain between 2002 and this year in the proportion of GCSE exam entries awarded getting a grade C or better.
They were up 11.3 percentage points, from 51.1% to 62.4%.
The situation is the opposite of that seen with A-level grades last week.
In those results, the lowest proportion of A grades was in the North East, at 19.8% of entries - an increase of just 2.1 percentage points since 2002, the lowest rate of improvement across the regions.
Looking only at the top grades, A and A*, London made the biggest gain, up 5.4 points from 18% to 23.4%.
And that figure for 2008 meant it had almost the best outright performance, pipped only by the wider South East region on 24.3%.
The director general of the big AQA exam board, Mike Cresswell, said more research needed to be done on the causes of the regional variations.
But he said: "There are obvious differences of a social kind between the regions, differences in the organisation of education and type of institution.
"It's a melange of very complex causes."
He said it did not seem to be related to the type of institutions - because a similar picture was seen in the various types of school in any given region.
Independent sector worse
A separate analysis produced by the exam boards, under their umbrella organisation the Joint Council for Qualifications, looked at the same results in that way.
This showed that at grade C or above, the greatest improvement between 2002 and 2008 was in comprehensives, with a gain of 8.9 percentage points from 53.5% to 62.4%.
Next best were secondary moderns, gaining 7.7 from 46.1% to 53.8%.
Independent schools actually did less well on this measure, with 0.4 percentage points fewer exam entries awarded C or above in 2008 than in 2002: down from 90.8% to 90.4%.
In grammar schools the gain was marginal - but from a very high starting point: up 1.2 from 93.9% to 95.1%.