This study investigated the occurrence of mild modified measles cases during an outbreak in Niterói, RJ, Brazil by using RT-PCR on oral fluid samples. From August to December 1997 a total of 76 patients with rash were seen at the study sites. Confirmed diagnosis by serology was achieved in 47 cases: measles (39.5%), rubella (13.2%), HHV-6 (3.9%), human parvovirus B19 (3.9%), dengue fever (3%). For 19 of the 29 patients without a conclusive diagnosis paired serum and saliva samples were available for further tests. In four of them, measles virus RNA was detected by RT-PCR in saliva samples in the absence of specific IgM in serum samples. Vaccination histories obtained from three of the RT-PCR positive cases showed that individuals previously immunized can still be infected and contribute to the circulation of measles virus. This study demonstrated the usefulness of RT-PCR on non-invasive clinical samples for the investigation of measles cases.