A prospective study of 37 men and 32 women with culture-proved genital herpes simplex virus (HSV) infection was undertaken to characterize the clinical manifestations of the disease and effects of topical therapy with Nonoxynol 9, a nonionic surfactant active in vitro against HSV. The duration of pain, lesions, and virus shedding was significantly longer in initial than in recurrent genital HSV infection. Cervical HSV shedding occurred in 11 of 13 initial infections versus two of 19 recurrences (p less than 0.001). Type 1 HSV caused seven of 29 initial and none of 40 recurrent infections (p less than 0.01). Clinical recurrences within six weeks were detected more often in men than in women and were unrelated to the menstrual cycle. The influence of coitus on recurrence is unclear. Papanicolaou smears were transiently abnormal (Class II) in four (12.5 per cent) of 32 female patients. Evidence of herpesvirus was detected cytologic smears from only 28 (41 per cent) of 69 culture-proved external lesions and three (23 per cent) of 13 culture-positive cervices. Serology was also of limited diagnostic value except in testing paired sera from initial infections. The topical therapy had no beneficial effect.