Weight gain and statural growth are slow in the Kivu area (Zaïre), as compared with international reference curves. One hypothesis put forward to explain this well-documented but poorly understood fact is inadequate breast-feeding with early intake of foods responsible for the introduction of microorganisms with deleterious effects on the digestive tract. A study of intestinal mucosa specimens from 90 at least partially breast-fed infants (4 to 22 months) showed that the appearance of the jejunal mucosa was consistently abnormal, as compared with histologic standards used in Europe. Furthermore, intestinal disaccharidase activities were always abnormal; lactase activities were extremely low and alpha-glucosidase activities were reduced by one-third to one-half as compared with European reference values. Age apparently influenced the magnitude of these modifications, whereas the correlation with nutritional status was less clear; only a correlation between the serum albumin level and the microscopic appearance of jejunal biopsy specimens was found. Intestinal alpha-glucosidase levels were lower in those patients with giardiasis. These findings demonstrate that in Kivu early atrophy of the intestinal mucosa contributes, together with the protein-calory malnutrition, to the lower growth rates in infants.