To document the proportion of each herpes simplex virus (HSV) type in genital HSV infection and changes over time during a 10 year period. Retrospective comparative study in sexually transmitted disease (STD) patients with genital HSV infection at the outpatient clinic for STD, Haukeland Hospital, Bergen, Norway. HSV-2 was the major cause during the 80's, whereas HSV-1 constitutes a greater part of the cases during the 90's, especially in female patients and in the younger age groups with primary or initial disease, where HSV-1 is the causative viral type in up to 70-90% of the cases. The documented change from HSV-2 towards HSV-1 in cases of genital HSV infection may have implications as to prognosis, future usefulness of vaccines, present and future usefulness of new type-specific serological tests.