A total of 127 people died in hospitals in the past year as a result of avoidable errors, according to figures released on Wednesday.
This was up on 92 deaths during the previous year.
Half of the deaths were the result of suicide, according to a report on safety in public hospitals in the year to June.
The suicides were included because they were by people who were being treated by community mental health services - a responsibility of district health boards.
Almost all of the suicides in the past year involved outpatients, four of which occurred in hospitals.
Falls and clinical management failures, including treatment delays, were the other main problems.
Details issued by each district health board include matters such as a lack of supervision of a high-risk procedure ending in the death of a patient.
The organisation charged with improving safety and quality says mental health will be an area of focus.
Nigel Millar, a member of the Health Quality and Safety Commission, which released the report, says it poses special challenges.
Senior doctors and nurses say better sharing of information is needed to improve the care given to mental health patients.