Inflammatory fibroid polyps are lesions occurring in the submucosa of the gastrointestinal tract. These lesions have been identified by a variety of names, indicative of their uncertain histogenesis. Three cases were studied by light microscopy and cytochemistry and, in one case, by electronmicroscopy. The most characteristic feature consisted of concentric fibrovascular structures. Peroxidase reaction for muscle actin and Ulex europeus agglutinin was positive in the two main cellular components. Electronmicroscopy revealed that the two principal cell constituents were endothelial and myocytic cells. These observations support the conclusion that inflammatory fibroid polyps are lesions of vascular origin.