During fifty years weaning age of piglets decreased from, in former days, ten to twelve weeks till, nowadays, three to five weeks. Early weaned piglets frequently have diarrhea. When piglets had diarrhea after weaning, enterotoxigenic E.coli (ETEC) or rotavirus were generally detected in the faeces; however, they were also detected in faeces of piglets without diarrhea. So, other causal factors must be involved in the occurrence of diarrhea after weaning. Mortality, due to diarrhea, was associated with severe villus shortening and crypt deepening. Furthermore, weaning piglets was also associated with villus shortening and crypt deepening and giving supplementary feed during the suckling period was effective in preventing villus shortening after weaning. Giving piglets sow's milk immediately after weaning is also effective in preventing villus atrophy. As shown with a small intestine segment perfusion test weaning was also associated with a loss of net absorption in both uninfected and ETEC-infected small intestine. The loss of net absorption was less in piglets that were given supplementary feed during the suckling period. Net absorption in the small intestine increased significantly one day after a rotavirus infection and decreased significantly after subsequent infection with ETEC. Function of the large intestine also influences the severity of diarrhea in just weaned piglets. It may be concluded that weaning piglets at an age of three to five weeks changes the flora, the morphology, and the function of the intestine; these changes together result frequently in diarrhea.