Anyone with a MyFitnessPal account is being told to change their passwords following a hack.
Users have been emailed to say the app is working with leading data security firms to assist in their investigation.
The app also says they have notified and are coordinating with law enforcement authorities
Under Armour, who owns the app, has revealed 150 million users' details have been stolen.
It includes names, email addresses and scrambled passwords - but any payments info stored isn't affected.
The attack happened last month but they say they've only just realised.