Background: On Saturday May 4, 2013, a train transporting chemicals derailed in the village of Wetteren (Belgium) and caused a leak of acrylonitrile (ACN). Objectives: To assess the human exposure to acrylonitrile in the local population with the highest suspected exposure. Methods: Between May 18-25, 242 residents participated in the study. N-2-cyanoethylvaline (CEV), a biomarker that is highly specific for ACN exposure, was measured in the blood. To account for potential influence by smoking, cotinine was determined in the urine. Participants also filled in a short questionnaire. Results: In the evacuated zone, 37.3% of the non-smokers and 40.0% of the smokers had CEV concentrations above the reference values of 10 and 200 pmol/g globin, respectively, at the time of the train accident. Spatial mapping of the CEV concentrations depending on the residential address showed a distribution pattern following the sewage system. Discussion and conclusion: The train derailment resulted in a highly atypical sequence-of-events. In addition to exposure in the direct vicinity of the site of the train derailment, exposure also occurred via the sewage system, into which acrylonitrile had entered shortly after the accident.