Purpose: Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. We recently characterized the phenotypic [2-13C]uracil breath test (UraBT) with 96% specificity and 100% sensitivity for identification of DPD deficiency. In the present study, we characterize the relationships among UraBT-associated breath 13CO2 metabolite formation, plasma [2- 13C]dihydrouracil formation, [2-13C]uracil clearance, and DPD activity. Experimental Design: An aqueous solution of [2-13C] uracil (6 mg/kg) was orally administered to 23 healthy volunteers and 8 cancer patients. Subsequently, breath 13CO2 concentrations and plasma [2-13C]dihydrouracil and [2-13C]uracil concentrations were determined over 180 minutes using IR spectroscopy and liquid chromatography-tandem mass spectrometry, respectively. Pharmacokinetic variables were determined using noncompartmental methods. Peripheral blood mononuclear cell (PBMC) DPD activity was measured using the DPD radioassay. Results: The UraBT identified 19 subjects with normal activity, 11 subjects with partial DPD deficiency, and 1 subject with profound DPD deficiency with PBMC DPD activity within the corresponding previously established ranges. UraBT breath 13CO2 DOB50 significantly correlated with PBMC DPD activity (rp = 0.78), plasma [2-13C] uracil area under the curve (rp = -0.73), [2-13C]dihydrouracil appearance rate (rp = 0.76), and proportion of [2- 13C]uracil metabolized to [2-13C] dihydrouracil (r p = 0.77; all Ps < 0.05). Conclusions: UraBT breath 13CO2 pharmacokinetics parallel plasma [2- 13C]uracil and [2-13C] dihydrouracil pharmacokinetics and are an accurate measure of interindividual variation in DPD activity. These pharmacokinetic data further support the future use of the UraBT as a screening test to identify DPD deficiency before 5-fluorouracil-based therapy. © 2006 American Association for Cancer Research.