Context. - Genital ulcer disease has been epidemiologically linked as a risk factor in the transmission of the human immunodeficiency virus 1 (HIV- 1). While herpes simplex virus 2 (HSV-2) is the most common cause of genital ulcers, no study has systematically evaluated the frequency or titer of HIV- 1 virus in HSV-2 lesions. Objective. - To compare lesional HIV-1 RNA levels during and after genital HSV-2 reactivation and to evaluate the frequency, titer, and duration of HIV-1 RNA shedding in lesions due to HSV-2. Design. - Convenience sample. Setting. - Sexually transmitted disease research clinic at the university of Washington, Seattle. Patients. - Twelve HIV-infected men with a history of symptomatic HSV-2 infection who underwent daily sampling of genital lesions for HIV-1 RNA by polymerase chain reaction assay and HSV-2 by culture. Main Outcome Measure. - Detection of lesional HIV RNA and HSV-2. Results. - HIV-1 RNA was detected from lesional swabs in 25 of 26 consecutively studied HSV-2 episodes and on 67% of days in which genital lesions were noted. The HIV-1 RNA titers in lesional swabs exceeded 10 000 copies/mL of swab sample in 75% of samples (range, 2.2-3.2 x 105 copies/ml of swab sample). HIV-1 RNA in genital lesion swabs was seen in persons with high and low titers of plasma HIV-1 RNA and was not associated with plasma HIV-1 RNA levels. Conclusions. - HIV-1 virions can consistently be detected in genital ulcers caused by HSV-2, which suggests that genital herpes infection likely increases the efficiency of the sexual transmission of HIV- 1.