4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung carcinogen present in both unburned tobacco and cigarette smoke. The sum of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronides, referred to as total NNAL, is an established urinary biomarker of human NNK uptake. Metabolic activation of NNK to DNA adducts proceeds via α-hydroxylation pathways, and 4-oxo-4-(3-pyridyl)-butanoic acid (keto acid) and 4-hydroxy-4-(3-pyridyl)-butanoic acid (hydroxy acid) are the principal end products of these pathways in rodents and primates. The purpose of this study was to determine NNK metabolic activation in smokers, as measured by the sum of keto acid and hydroxy acid, relative to total NNAL. To specifically identify NNK-derived keto acid and hydroxy acid, which are also formed from nicotine, we added [pyridine-D4]NNK to cigarettes that were originally low in NNK, and measured the deuterium-labeled metabolites in the urine of people who smoked these cigarettes. The total amount of [pyridine-D4]keto acid plus [pyridine-D4]hydroxy acid averaged 4.00 ± 2.49 nmol/24 h, whereas the average amount of total [pyridine-D4]NNAL was 0.511 ± 0.368 nmol/24 h. The results of this study show for the first time that NNK metabolic activation is a quantitatively significant pathway in smokers, accounting for ∼86% of total urinary excretion of NNK metabolites. The large interindividual variation in the excreted [pyridine-D4]keto acid and [pyridine-D4]hydroxy acid among 20 smokers strongly supports our hypothesis that some smokers activate NNK more extensively than others and that the ratio between biomarkers of metabolic activation and detoxification at a given dose of NNK could be a potential indicator of cancer risk.