An "ongoing rise" in complaints about the health service in Wales "is concerning", Wales' public services watchdog has warned.
Ombudsman Nick Bennett said he dealt with 863 complaints about the NHS over the last year - up 8%.
In his annual report, he singled out the north Wales health board as a "significant factor" for complaints.
The Betsi Cadwaladr University Health Board said it "valued" feedback and is reviewing how it deals with concerns.
Delivering his report, Mr Bennett said the number of complaints about health issues now accounted for 38% of his case work, a two point rise over since 2015-16.
"The ongoing rise in complaints about NHS bodies is concerning," said Mr Bennett.
"A significant factor is the high volume of complaints received about Betsi Cadwaladr University Health Board and my improvement officer will continue to work with the health board to ensure continued learning."
Million A&E visits
The Welsh Government said the increase in complaints should be put in perspective, and represented a rise of 65 additional health related complaints in the year.
"It's important to remember that every year the Welsh NHS deals with around 18 million contacts in primary care, three million outpatient attendances each year, one million A&E attendances and 750,000 admissions to hospital," said a government official.
"All NHS bodies in Wales are working hard to prevent complaints and to deal with them appropriately when they arise.
"We welcome the ombudsman's assistance through the improvement officers he has put in place."
The ombudsman has also assigned improvement officers to four other health boards: Abertawe Bro Morgannwg, Aneurin Bevan, Cwm Taf and Hywel Dda.
But Mr Bennett said a seminar held with health boards showed it was clear health board staff "are keen to strengthen governance, training and data collection arrangements to improve complaints handling".
During 2016-17, the ombudsman published seven special reports on public service complaints.
"All but one of the investigations were health related and worryingly, three of the cases were against the same hospital - Ysbyty Glan Clwyd run by Betsi Cadwaladr University Health Board," wrote Mr Bennett in his report.
The cases included:
- A patient with a history of chronic renal failure who waited 12 hours to see a consultant - and died a few hours later
- A cancer patient who was left waiting 132 days to receive his first treatment
Betsi Cadwaladr said the feedback provided by the ombudsman and patients was "an opportunity to learn and improve".
"We are working hard to make sure we respond to complaints in a timely manner and this includes the recently introduced Patient Advice and Support Service in Glan Clwyd Hospital," said a spokesman.
"It is important that we learn from when things go wrong so that we can make things better now and in the future."
Overall, the watchdog said his office had received 2,056 new complaints for 2016-17 over public services in Wales and a further 236 code of conduct complaints against local government councillors.
According to the ombudsman's office, the number of inquiries and complaints has increased by 75% in a period of six years.
Mr Bennett said the increasing workload demonstrated the need for legislation in a new draft Ombudsman bill being considered by assembly members.
"I am confident that if passed, the new legislation will allow cycles of poor service delivery to be spotted more easily and dealt with greater efficiency," added Mr Bennett.