Migrant farmworkers should be considered a vulnerable population because they work in a hazardous industry, are often members of an ethnic minority, have known difficulty in accessing health care, and are often of lower socioeconomic status. For these reasons, too, it is extremely important to conduct health-related research with this often-underserved group. However, because migrant farmworkers are vulnerable, investigators must be especially vigilant in protecting them from the potential harms of research and in ensuring that the special ethical issues that arise in research with this population are identified and addressed for every project. In response to the National Cancer Institute's concerns about the feasibility of conducting epidemiologic studies among migrant farmworkers, researchers undertook four feasibility studies near the Texas-Mexico border. Each study raised different, complex ethical questions that challenged the investigators, but whose resolution turned out to be crucial to the success of the studies.