This paper describes results of a qualitative study that explored barriers to research participation among African American adults. A purposive sampling strategy was used to identify African American adults with and without previous research experience. A total of 11 focus groups were conducted. Groups ranged in size from 4–10 participants (N=70). Mistrust of the health care system emerged as a primary barrier to participation in medical research among participants in our study. Mistrust stems from historical events including the Tuskegee syphilis study and is reinforced by health system issues and discriminatory events that continue to this day. Mistrust was an important barrier expressed across all groups regardless of prior research participation or socioeconomic status. This study illustrates the multifaceted nature of mistrust, and suggests that mistrust remains an important barrier to research participation. Researchers should incorporate strategies to reduce mistrust and thereby increase participation among African Americans.