In a study to determine the herpes simplex virus (HSV) type isolated from patients with primary (first-attack) genital herpes and to identify any clinical or epidemiological features that might influence the reported incidence of any particular viral type, there was almost an equal incidence of HSV-1 and HSV-2 isolated from women (48% versus 52%). In the case of women, the initial clinical diagnosis was correct in 97% with HSV-1 and 95% with HSV-2 infection. Amongst men, HSV-2 was isolated more frequently than HSV-1 (71% versus 29%). However, the initial clinical diagnosis in men was correct in only 53% with HSV-1 and 83% with HSV-2 infection. These results suggest that in primary genital herpes the incidence of HSV-1 and HSV-2 is probably equal, while in men the reported incidence of HSV-1 may be affected by underdiagnosis giving a falsely high incidence of HSV-2 infection.