NHS safety lapses in Wales killed 191 patients last year, up from 129 in 2005–6, according to figures obtained by Lib Dem spokesman Peter Black.
The number of so-called patient-safety incidents every year has more than doubled from 27,842 to 56,289 in the same three-year period, he said.
These include occasions such as when medical errors, abuse or bug outbreaks could have or did cause patients to be injured.
Thirteen patients died while being admitted, transferred or discharged – including patients who went missing. Self-harm accounted for the highest number of deaths – 39 last year.
Mr Black said: “The rise in deaths due to errors within the NHS is shocking given that many of these deaths are avoidable.
“Although we accept that a lot of incidents cannot be avoided, many can, and there needs to be better management and control to reduce the number of accidents and deaths.”
The increase is due to improved reporting within the health service, said Welsh NHS Confederation director Mike Ponton.
He said: “The NHS has worked extremely hard in recent years to create a culture of openness, where incidents are reported, thoroughly investigated and learnt from.”
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