Sixty-six source contacts of index patients with first-episode genital infection caused by herpes simplex virus (HSV) were evaluated for evidence of current or past HSV infections. Forty-three source contacts (65%) reported a history consistent with previous recurrent HSV infection or were experiencing a first episode of genital herpes when initially examined. However, 60% of these 43 contacts were not aware that they had transmissible HSV infection. Twenty-nine (67%) of the 43 individuals had had recent sexual contact with an index patient when lesions were present. All of the remaining 23 source contacts, who were without a history of symptoms consistent with HSV infection, had detectable neutralizing antibody to HSV; HSV type 2 was isolated from the cervix of two of these asymptomatic source contacts. Efforts to identify individuals with undiagnosed genital herpes and to instruct these individuals concerning the risk of disease transmission in the presence of lesions are needed if the rate of transmission is to be decreased; however, methods designed to decrease the rate of transmission by asymptomatic individuals must also be evaluated.