Epidemiological studies are increasingly collecting buccal cells and other sources of DNA for genetic analysis. However, high refusal rates raise concerns about possible selection bias. This study examines the subject characteristics associated with refusal or failure to provide a buccal cell sample. Subjects were male farmers in the Agricultural Health Study, which is being conducted in Iowa and North Carolina. As part of a 5-year follow-up, cohort members were contacted by telephone and asked to participate in a telephone interview and to consent to providing a buccal cell sample using a kit that was mailed to them. Demographic, lifestyle, disease, and occupational characteristics were compared between consenters who returned a sample ("compliers"), nonconsenters ("refusers"), and consenters who failed to return a sample ("noncompliers"). Compliers (n = 8794), refusers (n = 3178), and noncompliers (n = 3008) were quite similar, although compliers tended to be slightly older. Although some significant differences between these groups were observed, the magnitude of these differences was generally small, usually no more than a few percentage points. In conclusion, this study found little difference between male farmers who agreed to provide buccal cell samples versus those who either refused to provide a sample or who agreed but failed to return the sample. Observed differences were typically small and would be unlikely to compromise etiologic associations identified in such a prospective study. In short, there appears to be little selection bias in the Agricultural Health Study buccal cell collection process, further supporting the use of such mailed collection kits in epidemiological research.