The histological aspect and the mitotic index of jejunal biopsies were studied in 8 normal infants, in 18 marasmic infants, and in 10 infants with kwashiorkor. The mucosa of the marasmic infants was similar to the normal controls, although thinner. In kwashiorkor the mucosa was similar to celiac disease. In marasmic infants fed on the same diet, the mitotic index is significantly lower in infants who do not show weight increase when compared with those who do increase in weight. In kwashiorkor the mitotic index is closer to, although also significatively lower than, normal. It is suggested that these differences are due to the low caloric intake in marasmus where the energy supply is not sufficient to maintain the rate of cell proliferation; a hormonal mechanism governed by the pituitary gland is also suggested. In kwashiorkor the histological aspect similar to celiac disease seems to indicate a common pattern of reaction of the jejunal mucosa due to different causes.