Viral strains obtained during the same period which were asymptomatically shed into the saliva of patients with oral cancer after surgery or isolated from the focus of patients with herpes simplex virus type 1 (HSV-1) infection after first visit to hospital were analyzed using molecular epidemiology. There were 10 subjects in each group. Viral strains used were from 3 to 5 strains (mean: 3.4 strains) in the malignant patients and from 3 to 8 strains (mean: 5 strains) in the herpetic patients. One of 10 patients with oral cancer showed a different DNA cleavage pattern in viral strains shed into saliva at the last day. In 10 patients with HSV-1 infection, all viral strains in each of the cases showed identical DNA cleavage patterns. These results suggested the existence of a latent multiple infection and a simultaneous reactivation of HSV-1.