Guidelines in most developed countries for testing pregnant women for HIV infection recommend that screening should be directed to groups with acknowledged risk factors for infection. Our prospective study of HIV seroprevalence among 2724 pregnant women in an inner-city area of Baltimore, USA, showed that if these guidelines had been applied only 57% of HIV-seropositive women would have been detected. By offering counselling and HIV testing to all pregnant women, the detection rate was raised to 87%. We conclude that screening directed at women who admit risk factors is not effective in identifying HIV-infected women and that routine HIV screening should be offered to all pregnant women.