A technique was developed to quantitate the absorption of ingested carbohydrate by means of continuous measurements of pulmonary H(2) excretion. This technique is based on the observation that H(2) is produced in the colon when carbohydrate is fermented by colonic bacteria, and this H(2) is then excreted by the lungs. The quantitative relationship of pulmonary H(2) excretion to unabsorbed carbohydrate was studied in nine subjects. After ingestion of 6.5, 13, and 26 g of lactulose (a nonabsorbable disaccharide), H(2) excretion increased linearly, averaging (+/-1 SEM) 13+/-3.5, 23+/-7.2, and 49+/-7 ml per 2 hr. Because of consistent individual differences in H(2) excretion per gram of lactulose, the variability of this linear response was less in a given subject, with the H(2) excretion after 6.5 g and 26 g lactulose dosages averaging 55+/-4.2% and 214+/-16% of that observed after the 13 g dose. It was further demonstrated with fecal homogenates, as well as in rats after direct intracecal instillation of carbohydrate, that there was no significant difference in the rate of H(2) formation from lactulose as compared with the normally ingested sugars. Thus, a subject's H(2) excretion after a 13 g dose of lactulose can be used as a standard to convert H(2) excretion after ingestion of other carbohydrates into grams of carbohydrate not absorbed. Application of this technique to seven partially gastrectomized patients indicated all subjects malabsorbed a portion of a 100 g dose of glucose whereas six of seven completely absorbed a 25 g dose. Malabsorption of physiologic quantities of various carbohydrates was clearly demonstrated in one subject. This technique appears to provide quantitative information on carbohydrate malabsorption not readily obtained by presently available techniques.