To determine the risk of transmission of genital herpes simplex virus (HSV) infection, we prospectively studied, for a median of six months, 38 couples who had been together for a median of 10 mo. In each couple, one partner had a history of symptomatic genital herpes and one did not. At entry, of the 38 asymptomatic, exposed partners, 21 were seronegative, and results of western blot analysis showed that seven had antibody to HSV type 1 (HSV-1), four to HSV type 2 (HSV-2), and six to both HSV-1 and HSV-2. One of the 28 exposed partners without antibody to HSV-2 at enrollment asymptomatically acquired HSV-2 infection, but four of 10 with antibody to HSV-2 at enrollment developed culture-proven HSV-2 infection during follow-up. Restriction endonuclease analysis of DNA from paired isolates revealed identical strains in three couples and different strains in one. In this group of asymptomatic sex partners of persons with genital herpes, asymptomatic and unrecognized acquisition of HSV-2 infection was common, but more than half of the exposed partners remained free of HSV-2 infection after a median of 16 mo of sexual contact.