Complaints about NHS Highland have risen by almost 90% in just five years.
New figures show the number of grievances with the north health board have soared from 389 in 2011-12 to 737 in 2015-16.
NHS Highland also has one of the slowest response rates to complaints in the country, with just 39% handled within the 20 day target time.
In contrast, NHS Grampian’s complaints remained relatively static over the five-year period, with 17 more complaints in 2015-16 than in 2011-12.
The health board also posted a year-on-year drop in registered grievances, from 1,612 in 2014-15 to 1,397 in 2015/16.
Scottish Labour Highlands and Islands MSP Rhoda Grant said the figures were a “real cause for concern”.
But NHS Highland said the increase was partly down to “increased publicity promoting the complaints process”.
A spokesman for the health board said: “There has been an increase in complaints relating to waiting times for treatment. All letters sent to patients, who have breached the treatment time guarantee, included information on how the patient can make a complaint if they are not happy.
“Some complaints over the 20 working day response time were of a complex nature, involving meeting with patients or complainants, or were investigated through Significant Adverse Event Reviews.
“These obviously took longer and the response time was over the 20 working day timescales.
“The increase in volume of complaints and in the complexity of complaints have impacted on the board’s ability to meet the 20 working day response target.
“This is compounded because the board prefers to ensure quality of response and patient or complainant satisfaction over speed of response.”
