Summary The sera from patients with epidemic keratoconjunctivitis (EKC) in Japan, Italy, Switzerland, and North America regularly contain neutralizing antibodies to APC virus type 8, whereas such antibodies are absent from the general population in these areas. Paired sera from patients with EKC show a significant rise in neutralizing antibodies to APC virus type 8. These antibodies persist for only a limited period after the onset of the disease. The constant association of clinical EKC and antibodies to APC virus type 8 suggests that this agent (or antigenically related virus) may play a role in the etiology of this disease.