We prospectively followed 39 adults with concurrent primary herpes simplex virus (HSV) infection (12 with HSV type 1 and 27 with HSV type 2) of the oropharynx and genitalia, caused by the same virus in each person, to evaluate the influence of viral type (HSV-1 vs. HSV-2) and site of infection (oropharyngeal vs. genital) on the frequency of recurrence. The subsequent recurrence patterns of HSV infection differed markedly according to viral type and anatomical site. Oral-labial recurrences developed in 5 of 12 patients with HSV-1 and 1 of 27 patients with HSV-2 (P less than 0.001). Conversely, genital recurrences developed in 24 of 27 patients with HSV-2 and 3 of 12 patients with HSV-1 (P less than 0.01). The mean rate of subsequent genital recurrences (due to HSV-1 and HSV-2) was 0.23 per month, whereas the mean rate of oral-labial recurrences was only 0.04 per month (P less than 0.001). The mean monthly frequencies of recurrence were, in order, genital HSV-2 infections, 0.33 per month; oral-labial HSV-1 infections, 0.12 per month; genital HSV-1 infections, 0.020 per month; and oral HSV-2 infections, 0.001 per month (P less than 0.01 for each comparison). We conclude that the likelihood of reactivation of HSV infection differs between HSV-1 and HSV-2 infections and between the sacral and trigeminal anatomical sites. The sixfold more frequent clinical recurrence rate of genital HSV infections as compared with oral-labial HSV infections may account for the relatively rapid increase in the prevalence of clinically recognized genital herpes in recent years.