We tested the hypothesis that the stable isotope [(13)C]pantoprazole is O-demethylated by cytochrome P450 CYP2C19 and that the (13)CO(2) produced and exhaled in breath as a result can serve as a safe, rapid, and noninvasive phenotyping marker of CYP2C19 activity in vivo. Healthy volunteers who had been genotyped for the CYP2C19(*)2, CYP2C19(*)3, and CYP2C19(*)17 alleles were administered a single oral dose of [(13)C]pantoprazole sodium-sesquihydrate (100 mg) with 2.1 g of sodium bicarbonate. Exhaled (13)CO(2) and (12)CO(2) were measured by IR spectroscopy before (baseline) and 2.5 to 120 min after dosing. Ratios of (13)CO(2)/(12)CO(2) after [(13)C]pantoprazole relative to (13)CO(2)/(12)CO(2) at baseline were expressed as change over baseline (DOB). Maximal DOB, DOB(15) to DOB(120), and area under the DOB versus time curve (AUC(0-120) and AUC(0-infinity)) were significantly different among three genotype groups (CYP2C19(*)1/(*)1, n = 10; CYP2C19(*)1/(*)2 or CYP2C19(*)1/(*)3, n = 10; and CYP2C19(*)2/(*)2, n = 5) with predicted extensive metabolizers (EMs), intermediate metabolizers (IMs), and poor metabolizers (PMs) of CYP2C19, respectively (Kruskal-Wallis test, p < 0.01); linear regression analysis indicated a gene-dose effect relationship (r(2) ranged between 0.236 and 0.522; all p < 0.05). These breath test indices were significantly lower in PMs than IMs (p < 0.05) or EMs (p < 0.01) of CYP2C19. [(13)C]Pantoprazole plasma exposure showed significant inverse correlation with breath test indices in the respective subjects (Pearson r = -0.74; p = 0.038). These feasibility data suggest that the [(13)C]pantoprazole breath test is a reliable, rapid, and noninvasive probe of CYP2C19 and seems to be a useful tool to optimize drug therapy metabolized by CYP2C19.