Herpes simplex virus in the trigeminal ganglia of humans was studied in Philadelphia, Pennsylvania, United States of A merica, and in Kyoto, Japan. The prevalence of recurrent herpes labialis and of clinically latent herpes simplex virus within trigeminal ganglia was determined in inhabitants of the two cities. In addition, a comparison was made of the prevalence of mononuclear cell infiltration in the trigeminal ganglia of Americans and Japanese. Recurrent herpes labialis was found to be significantly less common in the Japanese city than in the American city. Herpes simplex virus was rescued less commonly from the trigeminal ganglia of cadavers in Japan than in America. The difference was significant. The frequency of mononuclear cell infiltration in the trigeminal ganglia of Americans and Japanese is not significantly different. These observations, as well as previously reported serological studies, suggest that despite the ubiquitous nature of herpes simplex virus in A merica and Japan, the Japanese have less clinically overt desease caused by this virus.