A retrospective review of genital herpes simplex virus (HSV) isolates collected in a university student health service over a 9-year period showed that an increasing proportion of isolates were HSV-1 rather than HSV-2. HSV-1 accounted for 78% of all genital isolates in this population by 2001, compared with 31% of isolates in 1993. BACKGROUND Herpes simplex virus (HSV) type 1 is usually thought to cause less than 30% of genital herpes infections in the United States, but the proportion of infections resulting from HSV-1 is increasing in some populations. OBJECTIVE The goal was to review the relative proportion of HSV-1 and HSV-2 as the cause of newly diagnosed genital herpes infections in a population of U.S. college students and to assess trends in the change of this proportion over time. METHODS Genital HSV isolates collected at a university student health service from 1993 to 2001 (n = 499) were reviewed retrospectively. Analyses included comparisons of isolates by HSV type, age group, and sex. RESULTS The proportion of newly diagnosed genital herpes infections resulting from HSV-1 increased from 31% in 1993 to 78% in 2001 (P <0.001, linear trend P <0.001). HSV-1 was more common in females than males, but increases were noted for both sexes. HSV-1 was more common in persons aged 16 to 21 than in persons aged 22 or older. CONCLUSIONS HSV-1 has become the most common cause of newly diagnosed genital herpes infections in this population of college students and reflects a reversal of the usual HSV-1/HSV-2 ratio.