Recent studies have shown increasing levels of unidentified fluorinated compounds, with unknown toxic effects to living organisms. These unidentified fluorinated compounds might be those novel replacements for PFOS and PFOA and some other related chemicals. These fluorinated substances have proven to be a challenge to identify and measure in blood samples and the number has been approximated to over 4700 compounds. Analysing the extractable organic fluorine (EOF) in samples and performing a mass balance analysis on measurable per- and polyfluoroalkyl substances (PFAS) to total EOF can provide an estimate on the levels of unidentified fluorinated compounds.Following the contamination of drinking water in Ronneby, a third of the population in the affected area were exposed to elevated levels of PFASs. PFHxS, PFOS and PFOA were measured in serum samples. The present study measured 11 target PFAS as well as EOF in whole blood samples from Ronneby (with known exposure) together with samples from three other cities in Sweden to preform a mass balance analysis in order to estimate the levels of unidentified fluorinated compounds. Results showed measurable concentrations of 11 PFAS in all cities included in this study, and mass balance analysis of EOF indicated the presence of unidentified fluorinated compounds in all samples, ranging from 47 % to 89 % among the samples from Ronneby and 11 % to 42 % among samples from Stockholm, Örebro and Malmö. The composition of 11 PFAS in human blood samples might indicate the source of exposure.