Utilizing the sulfur-containing gases of garlic as probes, we investigated the gut versus mouth origin of odoriferous breath gases. Five individuals ingested 6 g of garlic, and sulfur gases in mouth, alveolar air, and urine samples were measured. The mouth normally contained low concentrations of hydrogen sulfide, methanethiol, and dimethyl sulfide. Immediately after garlic ingestion, transient high concentrations of methanethiol and allyl mercaptan and lesser concentrations of allyl methyl sulfide (AMS), allyl methyl disulfide, and allyl disulfide were observed. With the exception of AMS, all gases were present in far greater concentrations in mouth than alveolar air, indicating an oral origin. Only AMS was of gut origin as evidenced by similar partial pressures in mouth, alveolar air, and urine. After 3 h, AMS was the predominant breath sulfur gas. The unique derivation of AMS from the gut is attributable to the lack of gut and liver metabolism of this gas versus the rapid metabolism of the other gases. Breath odor after garlic ingestion initially originates from the mouth and subsequently from the gut.