NHS Greater Glasgow and Clyde (NHSGGC) claims it is nearing a fix for what it described as a "major IT problem" that affected 709 patients.
On Tuesday evening, the health board released a statement stating that its technical staff were working to resolve the IT issue, resulting in a postponement of operations, chemotherapy sessions and outpatient appointments.
It said that the problem related to its networks and the way staff connect to clinical and administrative systems.
Yesterday morning, the firm said that despite the best efforts of its IT technical staff throughout the night, the problem had not yet been resolved. Although there was a short-term fix for chemotherapy patients.
Some hours later, a basic level of IT service to enable some clinical services was re-established, but the problem was still not fully resolved.
In total, NHSGGC postponed 485 outpatient appointments, 14 planned inpatient procedures, 43 day cases and 48 chemotherapy patient treatments.
The health board said that 7,400 patients have had their procedures and appointments maintained in the same period.
Robert Calderwood, NHSGGC's chief executive, apologised to patients affected by the IT glitch.
"I apologise unreservedly for the inconvenience this has caused to our patients who have had their procedures postponed and I will ensure that everything possible is done to get their treatment carried out at the earliest possible opportunity," he said.
Calderwood explained to BBC Radio Scotland's Good Morning programme today that the issue arose after Microsoft Active Directory became corrupted.
"As of 2am this morning, our technical teams, supported by Microsoft engineers, have re-run and re-profiled all of the IT systems that were affected and they are all operational as we speak.
"Clearly, as the hospitals and the departments busy up and more and more users come on the system, that will be the test.
"But as it stands at the moment, all systems are operational," he said.
He added that no data had been lost, and that any additional information would be manually added to the health board's computer systems.
The number of appointments that NHSGGC had to cancel had risen to 709, he said.