The clinical and virologic course of herpes simplex genitalis in women and men was examined in order to identify measurements useful in antiviral trials. Factors influencing the clinical course included initial disease versus recurrent disease, wet-skin versus dry-skin lesions, female versus male sex. Women with initial genital herpes had higher mean peak lesion virus titers than those with recurrent disease (10(4.5) pfu compared with 10(2.5) pfu) and excreted virus longer (13 to 15 days compared with 6 to 8 days). Men with recurrent lesions had higher mean peak virus titers than women (10(4.0) pfu compared with 10(2.5) pfu), but the duration of virus excretion was shorter (three to four days compared with six to eight days). There was pronounced variation in the clinical and virologic course of recurrent lesions among different patients and even within the same patient. These observations indicate several difficulties that must be considered in conducting careful antiviral trials in patients with herpes simplex genitalis.