Research by the Scottish Tories showed 4.2 per cent of meetings with patients were called off between April and June this year.
There was also wide variation in the percentage of appointments cancelled by different health boards. The Golden Jubilee National Hospital, which provides specialist heart and lung services in Glasgow, postponed 8.2 per cent of outpatient appointments while NHS Forth Valley postponed 2.2 per cent.
Jackson Carlaw, Scottish Conservative health spokesman and deputy leader, said: "In a system the size of Scotland's NHS everyone accepts there will be unforeseen circumstances resulting in the cancellation of appointments.
"However, the fact thousands of these are occurring on a monthly basis shows there's a severe and long-standing problem."
The total number of appointments cancelled between April and June was 19,461. A Scottish Government spokesman said: "Since 2008, the cancellation rate for appointments has actually fallen, and we are committed to ensuring this continues. Indeed, the recent report by Audit Scotland on waiting lists found improvements have been made to IT systems, with better controls and audit trails now in place."