What is the cause of a rash after measles-mumps-rubella vaccination?

Surveillance and laboratory confirmation of measles will increase in importance as Australia implements enhanced measles control. We describe a 17-month-old child with fever and rash after measles-mumps-rubella vaccination. Detection of vaccine-strain measles virus in his urine by polymerase chain reaction confirmed the diagnosis of a vaccine reaction rather than wild-type measles. We propose that measles virus should be sought and identified as vaccine or wild-type virus when the relationship between vaccination and measles-like illness is uncertain.