Tens or even hundreds of thousands of computer users could be unable to get online this summer.
The FBI is warning all computer owners to check for a type of malware, called "DNS Changer," similar to a virus.
PCs infected with DNS Changer were used by a group of hackers for fraud.
Federal agents say they've caught those responsible.
But a temporary solution keeping the infected machines on-line will go away in July, and users won't be able to connect to the internet.
You can check your computer by visiting FBI.gov finding DNS changer.