In 1988, the Los Angeles County Health Department conducted a blinded human immunodeficiency virus (HIV) seroprevalence study at a public prenatal (PN) and family planning (FP) center serving mostly Hispanic women in order to determine seroprevalence and to evaluate the center's targeted HIV screening program. Four women (0.13 percent) tested positive (3/1801 PN and 1/1167 FP). Three reported no risk factors; one reported a history of syphilis since 1978. Voluntary HIV testing was selectively offered to women who reported risk factors for HIV infection. Only 14 percent (96/685) of clients offered testing chose to do it: 28 percent (14/50) of clients classified as being at highest risk of infection, and 27 percent (16/59) of women who judged themselves to have some chance of being exposed to HIV. None of the four women who tested positive by blinded testing chose testing. While few women at this center were infected with HIV, higher risk women were not persuaded to be tested through a targeted screening program. Blinded HIV seroprevalence studies provide a tool for both tracking infection in a population and evaluating screening programs.