Differential sugar-absorption tests for measuring intestinal permeability for sugars have been studied in a variety of gastrointestinal diseases. Their use in general practice has been hampered by a lack of data on reference values and repeatability of the test and the laboratory assay. In this study, we determined the reference values of the sugar-absorption test, using lactulose and mannitol as probe molecules, for children and adults. The repeatability of the test is good; linear relationship: slope, 0.825 [95% confidence interval (CI), 0.571, 1.152); intercept, 0.005 (95% CI, -0.004, 0.010). The repeatability of the laboratory assay for the sugar-absorption test is excellent; linear relationship: slope, 1.014 (95% CI, 0.870, 1.094); intercept, 0.002 (95% CI, -0.005, 0.010). The validation of the sugar-absorption test makes the test useful as a simple, noninvasive, reliable intestinal permeability test for sugars, which can be of use for clinical practice. Taking possible interfering factors into account, the sugar-absorption test can be used as a diagnostic test for enteropathy of different etiologies and evaluation of therapeutic interventions in both children and adults. Studies with the sugar-absorption test may clarify the role of intestinal permeability in the pathophysiology of a variety of gastrointestinal diseases.