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The UH2/U is a plausible index of 5-FU response and toxicity.There is a case for therapeutic drug monitoring of 5-FU, also metabolites FUH2, FUPA and FβAL.An LC–MS/MS assay was developed for U, UH2, 5-FU, FUH2, FUPA and FβAL in plasma.The assay was shown to be sensitive, accurate, precise and robust.It was further validated in plasma from 10 cancer patients receiving 5-FU.The plasma 5,6-dihydrouracil/uracil (UH2/U) ratio is a possible phenotypic marker of dihydropyrimidine dehydrogenase (DPD) activity, hence an index of 5-fluorouracil (5-FU) response and toxicity. Studies have re-affirmed the value of 5-FU and 5,6-dihydro-5-fluorouracil (FUH2) for therapeutic drug monitoring (TDM). However, FUH2 has limited stability in plasma, necessitating expedited plasma separation and freezing, where routine compliance may not be easy. The metabolites α-fluoro-β-ureidopropionic acid (FUPA) and α-fluoro-β-alanine (FβAL) are stable in plasma and are probable candidates for TDM. This paper describes development, validation and application of an LC–MS/MS assay quantifying U, UH2, 5-FU, FUH2, FUPA and FβAL in human plasma.Extraction was by salt-assisted liquid–liquid extraction (LLE) in two-stages with pH adjustment. The supernatants were mixed, dried and reconstituted. Analytes were resolved on the Luna PFP (2) (150 × 2.00 mm, 3 μ) column by gradient elution and analyzed by tandem mass spectrometry via electrospray ionisation in positive polarity.The analytical response was linear (r2 ≥ 0.99) in the concentration (ng/mL) ranges: 50–10 000 for FβAL and FUH2, 50–5 000 for FUPA, 50–100 000 for 5–FU, 5–200 for U and 10–400 for UH2. Within- and between-run accuracy and precision were ≤ 10.2% and ≤ 9.8% respectively across the QC range and inclusive of LLOQ. The internal standard (IS) normalised matrix effects were within 93–112% with CV of ≤ 9.7% and normalised recoveries were within 91–107% with CV of ≤ 9.8%. This robust assay was successfully applied to samples from rectal and colorectal cancer patients (n = 10) on 5-FU. Deviations ≤ 2.0% from the mean values were observed when incurred samples were reanalysed.