1. Enterocyte development of microvillus structure has been measured in intestinal biopsies obtained from children suffering from coeliac disease, cow's milk protein intolerance and microvillus atrophy, and the results compared with similar measurements carried out in control children. 2. All types of enteric disease caused a significant 30% reduction in the length of microvilli present on undifferentiated basal crypt enterocytes, here referred to as potential stem cells. 3. Microvillus growth measured in control enterocytes took place mainly over the basal third of the villus. There was then little further change in structure during subsequent enterocyte migration to the villus tip. 4. Microvillus length in diseased tissue remained more or less constant during enterocyte migration to the crypt-villus junction. Microvillus length then decreased slightly during subsequent enterocyte migration over stunted villi. 5. The present results are discussed in relation to the supposed properties of potential stem cells. Comparisons are also made between profiles of microvillus development measured in healthy children and mature adults.