NHS hospital trusts in England reported 55 cyber attacks in 2016, according to data obtained by the BBC.
The figures come from NHS Digital, which oversees cyber security, and show an increase on 16 attacks in 2015.
NHS Digital said the figures showed a "rise in reporting, not necessarily a rise in cyber attacks".
But Oliver Farnan, from the Oxford Cyber Security Centre, said ransomware attacks had become more common.