Cell kinetics in the proximal jejunal epithelium were studied by the methods of Cairnie et al. and Wright et al. Seventeen children with untreated malabsorption syndrome and cow's milk protein intolerance (CMI) and 12 of these on a cow's milk free diet were compared with 47 children with untreated coeliac disease, with 15 of these on a gluten free diet, and with 15 controls. The total number of cells in the crypts of the patients with CMI was 1.8 times (P less than 0.001) and in patients with coeliac disease 2.4 times (P less than 0.001) that seen in the controls. During the elimination diet the total number of cells in the crypts returned to the level seen in the controls. The mitotic indices, both crude and corrected, were significantly higher (P less than 0.001) in untreated patients with CMI and those with coeliac disease than in the controls. During dietary treatment the indices fell, but not quite to the level of the controls. These small differences between the two groups may be due to the difference in the causative agents or to the different ages of the patients.