The disaccharidase activities of small bowel biopsy specimens obtained from 34 infants and children with various disorders were measured quantitatively. One example of a primary disaccharidase deficiency and several of a secondary type were encountered. The advantages of quantitative measurements in the recognition of these two groups are discussed. Small bowel biopsy, even in small infants, is felt to be a safe procedure, with considerable potential for study of many other metabolic aspects of intestinal mucosal function.