The literature on D-xylose testing has been reviewed, stressing advances in our understanding of absorption in general (including D-xylose absorption), the relationship of D-xylose testing to the development of excellent serologic tests for the diagnosis of celiac disease, the use of D-xylose testing in the evaluation of diarrhea in acquired immunodeficiency syndrome, new information on breath testing for the evaluation of malabsorption, and recent information on the understanding of D-xylose absorption compared with transcellular vs. paracellular transport. The authors suggest ways in which D-xylose testing might be employed in malabsorption or diarrhea evaluations, including some algorithms.