||Debates on human experiments in developing countries focus on ethical principles such as informed consent, accountability, involvement of the concerned communities, and the improvement of local health services. Public health specialists who conducted human experiments in Rio de Janeiro (1902-1905) and in Guatemala (1947-1948) believed, however, that they were acting in the best interests of local populations, were aware of the importance of informed consent, were closely collaborating with local health professionals, and were contributing to the development of local health structures. Nevertheless, their investigations went dramatically wrong. An initial desire to conduct ethically and scientifically sound studies was undermined by pressure to obtain results and to save the researchers' initial investment, the possibility of freely using hospitalized patients as experimental participants, uncritical help from local professional elites, and structural pitfalls of experimenting with severely deprived people. These elements can still be found in trials of preventive methods in the Global South.