Experimental investigations have shown alterations of the ileal mucosal surface after specific fat diets resembling early changes in Crohn's disease. An animal experiment in pigs has been conducted. After creation of an anisoperistaltic segment these were fed either a specific fat diet containing chemically processed, partially hydrogenated fats or a low fat control diet over a period of 3 months. Defined areas of the ileal lamina propria were examined by transmission electron microscopy with the underlying question to what extent ultrastructural alterations could be compared to Crohn's disease. In comparison to the control group these areas were characterized by a dense infiltration of "inflammatory" cells like lymphocytes, histiocytes, macrophages and plasma cells indicating a hyperplasia and activation of lympho-plasmocytotic cells. Additionally, a focal prominent infiltration of mast cells with degranulation was observed as well as a dilatation of axons with depletion of axonal organelles in half of the animals after fat-feeding. Compared to patients with Crohn's disease the results show obvious similarities. It is concluded, that chemically processed fats could cause direct stimulation of immunologically-specific and non-specific cells in the lamina propria mucosae or directly injure the intestinal mucosa with secondary infiltration of inflammatory cells into the lamina propria.