Bacille Calmette-Guérin (BCG) vaccines are widely used, even though estimates of efficacy have ranged from zero to 80%. BCG is a relatively safe vaccine, but it can cause disseminated infection, especially in immunocompromised hosts. Thus, the development of a more reliably efficacious and safer vaccine is important to the control of tuberculosis. The testing of any new vaccine in human populations presents a number of ethical challenges that must be addressed. These include (1) the appropriateness of conducting such trials in developing countries; (2) the use of a BCG-vaccinated population as the control group; (3) the provision of tuberculin skin-test screening and preventive therapy to study participants; (4) the involvement of various "communities" in the trial(s); (5) the structure and process of ethical review; (6) establishing an effective method of obtaining informed consent; and (7) the roles and responsibilities of researchers and others in ensuring that trial results are available to the study population after the trial ends.