Primary intestinal lymphangiectasia is a rare congenital condition associated with protein-losing enteropathy. Hypogammaglobulinemia and lymphopenia secondary to this condition are frequent but infectious complications are not. So far few immunological studies have been made in these patients. We report here the results of such a study carried out in two adolescents. Both patients presented with a dramatic decrease in serum gammaglobulins, especially IgG and IgA, and in peripheral blood lymphocytes, especially CD4 T helper cells. From a functional standpoint, the proliferative response to certain mitogens was reduced. A decrease in in vitro production of immunoglobulins by B lymphocytes may be due to a faulty T/B cell cooperation. Histological examination of duodenal biopsy specimens revealed a decreased number of intraepithelial lymphocytes. Colonoscopy revealed nodular lymphoid hyperplasia in the terminal ileum, confirmed by endoscopic biopsy. The role of these abnormalities in the development of infectious complications and lymphoma is underscored.