The case of a 60 year old man in whom diarrhea, steatorrhea and protein losing enteropathy developed one year after the diagnosis of Waldenström's macroglobulinemia is reported. Histological study of multilevel peroral biopsies of the small bowel mucosa showed the following features: a) lymphangiectasia, b) intralymphatic and interstitial deposits composed of lipids and immunoglobulin M of the kappa type (as demonstrated by specific histoimmunofluorescence), c) a mild lymphoplasmocytoid infiltrate, corresponding to the location of Waldenström's macroglobulinemia. The patient died six months later, in spite of improvement of his digestive tract functional abnormalities by a low-fat diet. Sixteen cases of small intestinal involvement have been reported during the course of Waldenström's macroglobulinemia, of which only one showed features similar to those found in our case. Lymphangiectasia appear to be due to mesenteric lymph node involvement by the tumoral process, and/or to increased blood viscosity; in turn the lymphangiectasia seem to be responsible for steatorrhea and plasma protein losses, since in our case both lipids and the immunoglobulin M-kappa were shown to cross the villous epithelium by immunohistochemistry.