Fluorine-19 nuclear magnetic resonance ( 19 F NMR) spectroscopy provides a highly specific tool for the detection, identification and quantification of fluorine-containing drugs and their metabolites in biofluids. The value and difficulties encountered in investigations on drug metabolism are first discussed. Then the metabolism of three fluoropyrimidines in clinical use, 5-fluorouracil, 5-fluorocytosine and capecitabine are reported. Besides the parent drug and the already known fluorinated metabolites, 12 new metabolites were identified for the first time with 19 F NMR in human biofluids. Nine of them can only be observed with this technique: fluoride ion, N-carboxy-α-fluoro-β-alanine, α-fluoro-β-alanine conjugate with deoxycholic acid, 2-fluoro-3-hydroxypropanoic acid, fluoroacetic acid, O 2 -β-glucuronide of fluorocytosine, fluoroacetaldehyde hydrate and its adduct with urea, fluoromalonic acid semi-aldehyde adducts with urea. This emphasizes the high analytical potential of 19 F NMR for the furtherance in the understanding of fluoropyrimidine catabolic pathways. 19 F NMR should also play a role in the therapeutic monitoring of FU and its prodrugs in specific groups of patients, e.g. hemodialyzed patients or patients with deficiency in FU catabolic enzymes.