Three hundred renal patients have been caught up in a data blunder at Hawke's Bay district health board.
The error meant some people due for recall appointments were not seen on time, the DHB said.
Acting Chief Medical Officer Russell Wills said the DHB unreservedly apologised to those people.
An investigation has been launched to find out why patient files were not being flagged correctly for follow-up.
The error dated back to March 2019, Wills said.
About half the affected patients had already been seen, with those at high risk given first priority.
"All patients have, and are still, being personally phoned to advise of the error, apologise and arrange an appointment as soon as possible," Wills said.
"It is our priority to contact these patients directly to ensure they receive the health care they need."
Local GPs have also been asked to contact the renal service if they're worried about anyone waiting longer than expected for a recall appointment.
So far, the DHB has identified one patient whose condition may have deteriorated because of the delay.