The number of patients waiting longer than 36 weeks to begin NHS treatment has more than tripled in the past four years, figures have revealed.
Numbers rose from 7,434 in September 2011 to 25,373 in June 2015.
The figures, obtained by Plaid Cymru, also showed the percentage of people waiting at least 26 weeks increased from 8.5% of all patients to 15.2%.
The Welsh government said half of patients wait less than 10 weeks.
Elin Jones, Plaid Cymru's shadow health spokeswoman, called the longer waiting times disappointing and unacceptable.
"These figures highlight that overall performance in the NHS is getting worse despite increases in the health budget. And the facts are that patients in Wales are waiting longer for treatment than in Scotland and England," she said.
A Welsh government spokesman said more than eight out of 10 people were waiting less than 26 weeks at the end of June 2015 while more than nine out of 10 were waiting less than 36 weeks.
"The Welsh NHS continues to work hard to make sure patients receive treatment in a timely manner and in order of clinical priority both for planned and urgent care," he added.