We here report on three patients with gastrointestinal symptoms in whom spirochetes were found in colonic biopsies. The patients, heterosexual adults, were not immunocompromised. Electron microscopy was performed on colonic biopsies from each of the three patients. Apart from the basophilic band consisting of spirochetes, the mucosa was normal in two patients on light microscopy and showed mild inflammation in the other one. However on electron microscopy there was invasion of the colonic epithelial cells, macrophages, goblet cells and Schwann cells by spirochetes, and stunting of the microvilli. The spirochetes conformed to the morphology of Brachyspira aalborgi, and no other infective etiology or pathology could be identified in these patients to account for their symptoms. Since the clinical significance of intestinal spirochetosis is uncertain, antibiotics were not administered to any of the three patients and all three improved symptomatically with non-specific treatment. Intestinal spirochetosis, previously thought to be non-invasive and non-pathogenic in humans, may be invasive and may be the cause of gastrointestinal symptoms in some patients.