The objective was to identify abdominal lymphatic malformations in pediatric patients with protein-losing enteropathy after palliation of complex congenital heart disease with total cavo-pulmonary connection (TCPC). In 2006, we performed complete hemodynamic and laboratory workup and thoracic and abdominal MRT screens in three patients who newly presented with symptoms of protein-losing enteropathy. All three patients, aged 3, 5, and 7 years, showed excellent TCPC hemodynamics with central venous pressures of 10–13 mm Hg. None of the patients had right-to-left overflow. All three patients showed extensive thoracic and mesenterial lymphangiomatosis. One patient died after 18 months of therapy, which included long-term parenteral nutrition, somatostatin, subcutaneous heparin injections, and frequent albumin and immunoglobulin substitution. The other two patients are in stable condition. Lymphangiomatosis might play an unknown role in the pathogenesis of protein-losing enteropathy after TCPC. It remains unclear whether lymphangiomatosis is a primary congenital disease related to the cardiac disease or if it is triggered by repeated surgery or venous congestion. The presence of lymphangiomatosis should be given diagnostic and therapeutic consideration in TCPC patients in the future.