An accurate and improved HPLC method was set up to measure both dihydrouracil (UH2) and uracil (U) in plasma, and to assess their ratio. Analytes retention time, separation and peak purity were greatly optimized with a Hypercarb column and a diode array detector. U and UH2 limits of quantification were 1.25 and 0.625 ng/mL. U and UH2 within-day precisions were 0.9-2.3% and 0.7-5.6%. Between-day precisions were 1.3-5.3% and 1.3-7.1%. Accuracy was 0.1-6.1%. UH2/U ratio between-day variability was low, but ratio decreased from 02:00 p.m. This method is now used in practice to detect patients at risk of fluoropyrimidine toxicity and to individually adapt the dosage.