We studied the natural history of herpes simplex virus (HSV) infection and its association with specific serum antibody in a sample of 68 HIV-infected patients with a first episode of Pneumocystis carinii pneumonia at San Francisco General Hospital in 1986. Seroprevalence was 66 and 77% for HSV-1 and HSV-2 antibody, respectively, by immunoblot assay. Twenty-seven patients had 45 HSV outbreaks diagnosed during 739 patient-months of follow-up. Median frequency of recurrence resulting in a medical visit was once every 6.5 months, and median duration of treated outbreak was 10 days. Fourteen of 48 evaluable patients seropositive for HSV-2 had no outbreak of HSV during a median follow-up of 7.5 months. Our data suggests that neither frequency nor severity of HSV were substantially increased in this group of patients, despite severe immunosuppression caused by HIV. However, validation of these results by a prospective study is required.