Protein-losing enteropathy (PLE) is a serious complication after modified Fontan operations in patients with congenital heart diseases (1–5). High central venous pressure with consequent intestinal lymphangiectasia has been suggested as the principal cause of PLE, but other potential contributing factors, such as inflammatory processes, have also been considered (1, 4, 5). Corticosteroid treatment for patients with PLE after a Fontan procedure has been reported as an effective therapy (1, 4, 5). Although the precise mechanism of action of corticosteroids is unclear, their effectiveness suggests that autoimmune and inflammatory processes may be one of the pathogenic factors of PLE. The local immune and inflammatory responses have not been fully investigated after Fontan procedure, and, to our knowledge, there has been no study of cytokine levels or electron microscopic findings in the small intestinal mucosa of patients with PLE after a Fontan operation. We report a boy who developed PLE after a modified Fontan operation. A jejunal biopsy was performed in this child before and after successful corticosteroid treatment of his PLE. Histologic analysis and measurement of the cytokine levels in the small intestinal mucosa were performed.