A 26-year-old male patient with mitral valve prolapse and HLA-B27 antigen received endodontic treatment for dental caries. Two weeks later fever, dysuria, diarrhea, sterile inflammatory arthritis of lower limbs, enthesitis, dactylitis, conjunctivitis, and uveitis consecutively developed. Blood culture performed at the time of active arthritis yielded Streptococcus viridans. He did not have any history of psoriasis, acute infectious diarrhea, chronic inflammatory bowel diseases, or sexually transmitted diseases. Laboratory studies also excluded the possibility of infections by human immunodeficiency virus, hepatitis B or C virus, chlamydia, and streptococci from the upper airway. This report indicates that Streptococcus viridans can be the triggering microorganisms of Reiter's syndrome in some circumstances.