Because of their dirt-, water- and oil-repelling properties, per- and polyfluoroalkyl substances (PFASs) are frequently used in a broad variety of consumer products. They have been detected in human samples worldwide. In Flanders, Belgium, the Flemish Environment and Health Studies (FLEHS) measured the levels of five PFAS biomarkers in four different age groups of the Flemish population and identified determinants of variability in exposure. Cord plasma or peripheric serum samples and questionnaire data were available for 220 mother-newborn pairs (2008-2009), 269 mother-newborn pairs (2013-2014), 199 adolescents (14-15 years old, 2010), 201 adults (20-40 years old, 2008-2009) and 205 adults (50-65 years old, 2014). Measured levels of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorohexane sulfonic acid (PFHxS) and perfluorononanoic acid (PFNA) in Flanders are in the middle or low range compared to concentrations reported in other Western countries. Levels of perfluorobutanesulfonic acid (PFBS) were below the quantification limit in 98%-100% of the samples. Despite decreasing levels in time for PFOS and PFOA, 77% of the adults (2014) had serum levels exceeding HBM-I values of 5 mu/L for PFOS and 2 mu g/L for PFOA. Beside age, sex, fish consumption, parity and breastfeeding, the multiple regression models identified additionally consumption of offal and locally grown food, and use of cosmetics as possible exposures and menstruation as a possible route of elimination. Better knowledge on determinants of exposure is essential to lower PFASs exposure. ; This paper is based on research conducted within the framework of the Flemish Centre of Expertise on Environment and Health, funded by the Environment, Nature and Energy Department of the Flemish government, the Department of Public Health and the Department of Science of the Flemish government. The views expressed herein are those of the authors and are not necessarily endorsed by the Flemish government. ; Colles, A (reprint author), VITO ...