We examined the digestive and absorptive function of the small intestinal mucosa in three patients with pernicious anemia to determine the functional correlates of the morphologic changes previously described. Digestive brush border enzymes (disaccharidases and leucyl-naphthylamidase) in jejunal biopsy specimens from the patients followed a normal distribution compared with those in the control group. With the exception of one patient with mild steatorrhea, the rest of the absorption test results were within the normal range. Jejunal perfusion studies, however, with glucose, sodium and water showed intestinal secretion of sodium and water, i.e., net movement of sodium and water from plasma to lumen, in the presence of normal glucose absorption. Follow-up studies in two patients after treatment with vitamin B12 did not reveal any significant improvement in the absorption rates from the pretreatment period. This abnormality of sodium and water transport in pernicious anemia represents another intestinal defect of a systemic disease which is not corrected by vitamin B12 replacement therapy.