OBJECTIVE: The objective of this study was to determine serum (perfluorooctanoate [PFOA]) in residents near a fluoropolymer production facility: the contributions from air, water, and occupational exposures, personal and dietary habits, and relationships to age and gender. METHODS: The authors conducted questionnaire and serum PFOA measurements in a stratified random sample and volunteers residing in locations with the same residential water supply but with higher and lower potential air PFOA exposure. RESULTS: Serum (PFOA) greatly exceeded general population medians. Occupational exposure from production processes using PFOA and residential water had additive effects; no other occupations contributed. Serum (PFOA) depended on the source of residential drinking water, and not potential air exposure. For public water users, the best-fit model included age, tap water drinks per day, servings of home-grown fruit and vegetables, and carbon filter use. CONCLUSIONS: Residential water source was the primary determinant of serum (PFOA).