The way some A-level downgrades have been decided is "unbelievable", a Labour Member of the Senedd has said.
Swansea East MS Mike Hedges said he knew of pupils receiving a lower than predicted grade "without any samples of their work being checked".
He and his Labour colleague David Rees are calling for a review of the system after 42% of final grades were lowered from teacher estimates.
Education Minister Kirsty Williams said the system was fair and "very robust".
A Welsh Government spokesman said it expects to provide more information on the appeals process next week.
Mr Rees, who represents Aberavon, told BBC Wales he was aware of individual cases where downgrading did not "reflect the students' ability".
"We do need to review it and assure ourselves no student has been unfairly treated," he said.
Cardiff Central MS Jenny Rathbone said information she had been given suggested large schools serving disadvantaged pupils had been "hammered by inexplicable reductions in grades".
And Mr Hedges said pupils should be given their predicted grades "unless there is a good reason not to".
"[It is] unbelievable that they downgraded candidates without seeing any of their work," he said.
"There needs to be an immediate review - it is young people's futures we are talking about."
Labour's Lynne Neagle MS, who chairs the cross-party Senedd education committee, said the process was a "matter of significant concern for learners, their parents and carers, and education staff".
"We are considering our next steps as a committee carefully, and we are committed to holding the government and public bodies to account on the approaches adopted in response to this pandemic," Ms Neagle said.
"Fairness, accuracy and clarity must be provided for all our young people who were due to take examinations this year.
"Their wellbeing and their ability to plan for their future learning and careers must be at the centre of our thinking."
Another Labour MS, Pontypridd's Mick Antoniw, said he was meeting head teachers in his area to "discuss their concerns".
The Labour-led Welsh Government announced on Thursday students unhappy with their grades will be able to appeal free of charge.
Plaid Cymru had been calling for that, and the party's education spokeswoman Sian Gwenllian said the appeals procedure must now be "robust and independent".
"GCSE pupils awaiting their results next week must be treated with more compassion than the students obtaining their A-level grades and any changes need to be announced sooner rather than later," she added.
Appeal grounds
A Welsh Government spokesman said about 94% of the grades were the same or within one grade of those students were predicted to achieve.
"This is before any adjustments are made as a result of the AS [level] floor announced yesterday by the minister for education," he added.
"We have also asked Qualifications Wales to consider whether the grounds for appeal can be broadened for all A-level, AS-level and GCSE qualifications and we expect to provide more information on this next week.
"No student from Wales should be disadvantaged against other UK students.
"We are pleased that, following the publication of the results today, UCAS have reported the highest number of Welsh learners being offered a place at university since 2011."