The assessment of D-xylose absorption by measurement of urinary excretion is subject to several sources of error. An investigation has been made into the use of the blood levels obtained after a 5-g dose as an assessment of malabsorption. We have shown that optimal discrimination between normal and impaired xylose absorption is achieved by measuring blood xylose levels 1 hr after oral administration and then correcting the results to a constant body surface area. The reference range was 0.65 to 1.33 mmoles per liter for values corrected to 1.73m2. In a prospective study of the value of this modification in investigating malabsorption, the incidence of false-negatives and false-positives was found to be 4.8 and 2.2%, respectively. The test is of particular value in the elderly because surface area corrected 1-hr blood xylose levels are independent of age, eliminating the expected and observed declines in renal function and urinary xylose excretion in this age group.