Operations have been cancelled after hospitals and GP practices run by NHS Lanarkshire were hit by a cyber-attack.
The health board confirmed malicious software had been detected in IT systems, causing appointments and procedures to be postponed.
It said it “took immediate action” to put a solution in place and minimise disruption. Patients are asked not to attend hospital unless it is essential.
It is unclear whether the health board was deliberately targeted.
NHS Lanarkshire was one of the worst-hit health authorities in Scotland in the widespread cyber-attacks in May.
However, a spokeswoman for the health board said the current issues were “not on the same scale”.
She said a number of appointments and procedures had been postponed as a result of the current issues, but there were “no concerns around emergency treatments”.
The board’s chief executive Calum Campbell said: “We have detected some incidences of malware and took immediate action to prevent this spreading while we carried out further investigations.
“We are currently putting in place a solution from our IT security provider.
“While the issue is being resolved, our staff have been working hard to minimise the impact on patients and we apologise to anyone who has been affected.”
NHS Lanarkshire’s hospitals include Monklands, Wishaw General and Hairmyres.
The health board has urged people to consider whether they definitely need emergency care before attending A&E.
Dr Jane Burns, dedical director for the acute division said: “I would ask that patients do not attend our hospitals unless it is essential.
“If you do turn up at A&E and do not require emergency care you may be sent away from the department or you may experience a lengthy wait.
“Emergency care will still be provided for those who do require to be seen.”