Summary: Patients attending a referral sexually transmissible diseases clinic at Muhimbili Medical Centre in Dar-es-Salaam during the period 1989 to 1993 were examined for herpes simplex virus type 2 (HSV-2) antibodies. An ELISA technique, using glycoprotein G of HSV-2 as antigen, was used to test 294 patients' sera. Of these, 126 sera were HSV-2 positive, while 168 were negative, yielding an overall HSV-2 prevalence of 42.9%. Sixty-three per cent of the women and 35.5% of the men were HSV-2 positive. Seropositivity rose from 8.7% in the youngest men to 61.5% in the oldest male age group, while even the youngest women aged 20 or less had an HSV-2 prevalence of 55.6%. There was a significant positive association between HIV and HSV-2 seropositivity ( P =0.0006), most pronounced among the youngest women. There was no over-representation of HSV-2 positivity among patients with genital ulcer disease, indicating that other causes of this disease could be more common than HSV-2 in Tanzania.