It is well known that spirochetes may be found in the digestive tract of various species, including humans. We describe a homosexual man with long-standing diarrhoea, probably caused by intestinal spirochaetosis. The diagnosis was based upon light microscopic demonstration of a thin haematoxyphilic bandlike layer covering the intestinal surface epithelium, which is an established diagnostic characteristic of intestinal spirochaetosis. The spirochaetosis was further confirmed by electron microscopy. The pathogenetic significance of spirochetes in human intestinal disease is not fully understood. However, intestinal spirochaetosis should be suspected in patients with long-standing diarrhoea. Symptomatic spirochaetosis in the gut responds well to treatment with metronidazole. The spirochetes of the human digestive tract constitute a newly defined genus of the family Treponemataceae.