Dentists’ bank account numbers and correspondence with a trade body are feared to have been stolen by hackers.
The British Dental Association has told its members that it is still not sure exactly what was accessed in a breach on 30 July.
A spokeswoman told the BBC it was possible that information about patients was exposed, but was vague about the potential context.
The BDA’s website has been offline since the attack.
It has urged members to be cautious of any correspondence claiming to be from a bank following the incident.
The organisation is contacting those it thinks had data compromised.
In an email to members, the BDA referred to “logs of correspondence and notes of cases” as being among the information it said it had to be assumed stolen.
A spokeswoman initially told the BBC that the notes related to insurance claims, which could include patient information.
But she later clarified this and said they referred to the BDA’s “work as a trade union representing member interests” without providing further information.
The BDA does not hold full patient records.
Its chief executive Martin Woodrow said that an investigation was still underway.
“Owing to the sophistication of these criminals, we cannot, as yet, confirm the full extent of information that has been accessed,” he added in the email memo.
“We are devastated and apologise unreservedly for this breach.”
Source: BBC News