Nearly 300,000 outpatient appointments were missed in Wales' NHS hospitals in 2016-17.
Health boards claim it cost more than £36m, although similar calculations have been questioned in the past, because clinics are sometimes overbooked.
Dr Mike Fardy from the University Hospital of Wales said it was "infuriating" for medics and patients.
Some health boards use reminder texts to tackle the problem.
The percentage of missed appointments has hovered between 8.6% and 9.5% over the past six years, but has dropped from a high of 324,415 in 2013-14.
Cardiff and Vale health board had the highest number and percentage of missed appointments in 2016-17 with more than 71,000 no-shows - 11% of its total.
It has started a pilot project so patients get a text, repeating the date and time and telling them the cost to the NHS if they do not show up.
Tina Ball, who manages the project, said missed appointments meant a lot of work had gone to waste.
"It's the processing of the referral when it comes in, placing [it] on the waiting list, the appointment is made, we have to look for the notes and then it's the clinician's time," she said.
"You equate all of that to an appointment and it costs around £160."
Denis Williams, ophthalmology surgery directorate manager at the health board, said his department saw about 12% of all appointments missed.
"That means down-time for surgeons and for theatre staff and an ability that we don't have to treat the [other] patients that need their surgery," he added.
However, Andrew Street, a professor at London School of Economics, previously told BBC Radio 4's More or Less programme that while missed appointments were a problem, solving them will not necessarily save as much money as has been suggested.
Responding to a previous NHS estimate which suggested the average cost of an appointment was £120, he said: "It's a massive overestimate of the true cost... A missed appointment costs closer to zero.
"That is only the true cost of missed appointment if staff stand around doing nothing if a patient doesn't turn up. Outpatients clinics tend to be overbooked."
Dr Fardy, a consultant maxillofacial surgeon in Cardiff, said: "It may be a small procedure but it's a valuable procedure that somebody else could have had.
"It is absolutely infuriating. I'm not being judgemental but if you tell us, then we have a list of people [other patients] who can come in at very short notice."
Aneurin Bevan health board said its text reminder service, launched in 2016, proved effective with the rate of missed appointments dropping shortly afterwards from about one in 10 to one in 20.
Its did-not-attend appointments fell by 8,591 from 2015-16 to 2016-17 - a notional saving of about £1.17m.
In 2015, the Welsh Conservatives mooted a £10 fine for patients who frequently missed appointments but the Welsh Government said it would be too complicated and GP bodies dismissed the idea.
- 2011-12 - 315,908 (8.9% of total)
- 2012-13 - 321,169 (9.5%)
- 2013-14 - 324,415 (9.5%)
- 2014-15 - 322,623 (9.5%)
- 2015-16 - 299,019 (9%)
- 2016-17 - 296,423 (8.6%)