The Florida Department of Health says four children who were diagnosed with measles were never vaccinated.
The health agency says the children in Sarasota County on Florida's Gulf Coast had close contact with each other but didn't elaborate further.
The Sarasota Herald-Tribune reports that health officials are trying to notify others who may have been exposed.
Measles is a highly contagious respiratory disease caused by a virus. It is easily spread by air droplets when infected people breathe, cough, or sneeze.
Federal officials declared the contagious virus had been eliminated in the U.S. in 2000.
But infections periodically occur nationwide, as the virus is still common in many other parts of the world. Travelers can bring measles into the country, where it can spread among people who are not vaccinated.