Large numbers of pregnant women in Africa have been invited to participate in studies on HIV infection. Study protocols adhere to guidelines on voluntary participation after pre-test and post-test counselling and informed consent; nevertheless, women may consent because they have been asked to do so without fully understanding the implications of being tested for HIV. Our studies in Nairobi, Kenya, show that most women tested after giving informed consent did not actively request their results, less than one third informed their partner, and violence against women because of a positive HIV-antibody test was common. It is important to have carefully designed protocols weighing the benefits against the potential harms for women participating in a study. Even after having consented to HIV testing, women should have the right not to be told their result.