SummaryProtein‐losing enteropathy (PLE) and edema are usually the most prominent clinical features in children with Ménétrier's disease. However, the changes in gastrointestinal mucosa that can cause PLE have not been described yet in children. We studied by electron microscopy the mucosa of the gastric fundus, which is the site where macroscopic changes are most prominent, in two children with Ménétrier's disease. We found that tight junction width was increased to 10.5 ± 0.94 nm (mean ± 1 SD) in one child and to 9.7 ± 0.7 in the other. Tight junction width returned to normal when PLE and edema subsided. These ultrastructural changes were similar to those described in adults with the disease, although the clinical course of Ménétrier's disease is very different in adults and in children. Both patients showed evidence of cytomegalovirus (CMV) infection, as indicated by increasing IgG antibodies against the virus or recovery of the virus in the urine. Although Helicobacter pylori was found in the antral mucosa of one patient, the clinical course of the disease was not related to this microorganism. We conclude that increased tight junction width plays a role in PLE seen in Ménétrier's disease in children and that CMV, rather than Helicobacter pylori, is associated with the disease.