Intestinal spirochetosis in humans (HIS) is a condition defined by the presence of a layer of spirochetes attached by one cell end to the colorectal epithelium. The pathologic significance of HIS is uncertain, but it has been linked to chronic diarrhea and other abdominal complaints. Two anaerobic intestinal spirochete species have been associated with HIS, namely Brachyspira pilosicoli and Brachyspira aalborgi. Brachyspira pilosicoli, which colonizes many animal species, is common (approximately 30%) in the feces of people from developing countries, including Australian Aborigines, and in HIV+ patients and male homosexuals in Western societies. It is also commonly seen attached to the rectal mucosa of homosexual males. In other groups in Western societies both the presence of B. pilosicoli in feces and histologic HIS are uncommon (approximately 1.5%). Brachyspira aalborgi is an extremely slow growing and fastidious spirochete, which previously had been isolated from an HIS patient in Denmark. Recent studies using polymerase chain reaction amplification of DNA from intestinal biopsies from a series of cases of HIS in the general Western population demonstrated that B. aalborgi, rather than B. pilosicoli, was the main spirochete species involved in these patients. This review outlines recent developments in the study of HIS and the two spirochete species, and identifies priorities for future research.