We investigated the possible contribution made by oropharyngeal microfloral fermentation of ingested carbohydrate to the generation of the early, transient exhaled breath hydrogen rise seen after carbohydrate ingestion. Ten subjects ate or were sham fed carbohydrate-containing meals with and without prior chlorhexidine mouthwash during serial collection of exhaled breath and mouth hydrogen samples. Meal ingestion and sham feeding both induced significant (p less than 0.01) elevations of breath and mouth hydrogen that were virtually abolished by prior chlorhexidine mouthwash. In 7 subjects, delivery of the meal directly into the stomach via an orogastric tube did not cause a breath or mouth hydrogen rise. Oral contents incubated anaerobically in vitro with carbohydrate generated hydrogen that was again inhibited by chlorhexidine. These studies indicate that fermentation of ingested carbohydrate by oropharyngeal bacteria can contribute significantly to measured breath hydrogen values soon after meal ingestion, and may introduce avoidable error into the interpretation of serial breath hydrogen data.