The diagnoses which may be arrived at by examination of peroral small bowel mucosal biopsy specimens are presented. Celiac sprue, unclassified sprue (refractory sprue), infectious gastroenterititis, stasis syndrome and kwashiorkor have a severe mucosal lesion. Other clinical conditions are required to establish the diagnosis in these diseases. A number of diseases have specific diagnostic features. Included are Whipple's disease, abetalipoproteinemia, collagenous sprue, primary intestinal lymphoma, eosinophilic gastroenteritis, giardiasis, coccidiosis, strongyloidiasis, lymphangiectasis and the intestinal immunodeficiency diseases. Mucosal abnormalities may be present in other diseases but the diagnoses are usually made on other criteria than small bowel biopsy. These include vitamin B12 or folic acid deficiency, Crohn's disease, gastrinoma, acrodermatitis enteropathica, amyloidosis, chronic granulomatous disease, lipid storage diseases, histoplasmosis, capillariasis, cytomegalovirus infection, schistosomiasis and macroglobulinemia.