The efficacy of D-xylose testing in clinical situations has been reviewed in the light of recent kinetic studies. The standard 25-g D-xylose test in adults, based on analysis of 5-h urine collection and a 1-h serum sample, discriminates between normal subjects and patients with proximal small intestinal malabsorption with greater than 95% specificity and sensitivity. The 1-h serum level measured after administering this dose is also useful in evaluating malabsorption in patients with intermediate degrees of renal insufficiency and in the elderly. The 1-h serum test after administration of 5 g of D-xylose should be used in pediatrics and is greater than 91% sensitive and close to 100% specific. The [14C]D-xylose breath test with 1 g of D-xylose has been useful in identifying malabsorption caused by bacterial overgrowth in the small intestine.