Health chiefs at a clinic have admitted that frozen sperm samples of a number of patients were mistakenly destroyed.
Cardiff and Vale University Health Board made the admission after reports cancer patients could be among those affected.
Bosses at the South Wales facility are understood to have dismissed fears over the ability of patients to parent children.
But the Welsh Assembly Government is calling for urgent action to improve standards.
Concerns had been raised that men with conditions such as testicular cancer had lost the chance to father children in future.
Such patients are among those opting to use the clinic before starting treatments which could leave them infertile.
The health board, which hosts the IVF Wales clinic, claims lessons have been learned from the problems.
In a statement it said work is continuing to identify affected patients, none of whom have yet needed to be contacted.
The revelation follows the unexplained resignation of Janet Evans, head of IVF Wales, and another senior staff member earlier this year.
Details of the clinic's problems emerged in a report by the Human Fertilisation and Embryology Authority, which regulates IVF centres in the UK.