The aim of this study was to determine the acceptability of HIV counseling and testing and participation in a mother-to-child HIV-1 transmission intervention study using antiretroviral therapy in Dar es Salaam, Tanzania, one of the sites for the Joint United Nations Program on AIDS (UNAIDS) multicenter Petra trial. HIV testing was offered to all pregnant women who visited three prenatal clinics in Dar es Salaam before 34 weeks' gestation. Group or individual pretest counseling was performed by trained midwives. Laboratory diagnosis of HIV infection was based on two sequential anti-HIV enzyme-linked immunosorbent assays. Posttest counseling was given 2 weeks later to women who wished to know their HIV status. HIV testing was offered to a total of 10,010 pregnant women from June 1996 to May 1998, of whom 76.4% (7647 of 10,010) agreed to be tested. The prevalence of HIV-1 infection was 13.7% (1050 of 7647). Overall, 68.1% (5205 of 7647) returned for their results. Of the HIV-1-seropositive respondents, 27.4% (288 of 1050) agreed to participate in the Petra trial after fulfilling the eligibility criteria. Only 16.7% (48 of 288) of the enrolled women disclosed their positive HIV serostatus to their sexual partners. The main reasons for not disclosing the HIV serostatus were fear of stigma and divorce. Sixty percent (29 of 48) of the informed sex partners agreed to be tested for HIV and 69% (20 of 29) tested HIV seropositive. Pregnancy recurrence rate was 4.4 per 100 women years (18 pregnancies during 408 women years of follow-up) with 10 of 18 (55.6%) women not wanting to carry the pregnancy to term. In conclusion, this information is useful in planning intervention programs for prevention of mother-to-child HIV-1 transmission and it shows that improvements are required in counseling.