Ten pediatric patients investigated for chronic diarrhea, chronic weight loss, or failure to thrive were found on intestinal biopsy and/or in a duodenal aspirate to have Giardia lamblia. Serum immunoglobulin levels were normal or elevated in all patients. Three children had increased excretion of fecal fat and three other children had low D-xylose absorption. Jejunal biopsy specimens showed two severe, three moderate, and two mild morphological abnormalities, and three were normal. Except for lactase deficiency, disaccharidase activities correlated poorly with the severity of mucosal damage on biopsy. Steatorrhea was seen only with the more normal biopsies. Immunofluorescent staining of the biopsies for IgG, IgM, IgA, and secretory piece revealed no immune defects. Thus, there was no single malabsorption defect associated with giardiasis, and the specific defects did not necessarily correlate with morphological changes.