In July 1995 an outbreak of pharyngoconjuctivitis caused by adenoviruses occurred among athletes participating in a swimming contest in a town in southern Greece (Peloponnese). At least 80 persons displayed symptoms of the illness, with the predominant ones being high fever, sore throat, conjuctivitis, headache, and abdominal pain. Poor chlorination was probably the cause of the outbreak (residual chlorine <0.2 mg/l), as after hyperchlorination the spread of adenoviruses stopped. Rapid detection of adenoviruses in the municipal swimming pool water by nested polymerase chain reaction (PCR) amplification allowed quick control of the outbreak.