BACKGROUND N'-nitrosonornicotine (NNN), an esophageal and oral carcinogen present in tobacco products, has a chiral center in its structure. Of its two enantiomers, (S)-NNN exhibits higher tumorigenic potency than (R)-NNN. There is no information available on the levels of (S)-NNN in various tobacco products currently marketed in the United States. METHODS We used chiral gas chromatography analysis to determine (S)-NNN levels in a convenience sample of 37 tobacco products currently marketed in the United States: conventional smokeless tobacco, novel smokeless tobacco products, and cigarette tobacco filler. RESULTS Among all products analyzed here, (S)-NNN averaged 62.9 ± 6.3% (SD) of NNN. The absolute amount of (S)-NNN in conventional moist snuff averaged 1.26 ± 0.5 µg/g tobacco; in novel smokeless products 0.70 ± 0.2 µg/g tobacco; and in cigarette filler 1.36 ± 0.6 µg/g tobacco (all values are per wet weight). For each cigarette brand, the enantiomeric composition of NNN in cigarette smoke was similar to that of the corresponding tobacco filler. CONCLUSIONS Our results demonstrate that (S)-NNN is the predominant NNN enantiomer in moist snuff, novel smokeless tobacco products, and cigarettes currently marketed in the United States. Efforts toward the reduction of NNN in U.S. tobacco products should take into account its enantiomeric composition, with particular focus on (S)-NNN as a causative agent for esophageal and oral cancers associated with tobacco use.