Strains of Bacteroides fragilis which produce enterotoxin(s) (ETBF) have been associated with diarrhoeal diseases in young domestic animals and have also been isolated from humans with diarrhoea. We have determined epidemiologically that ETBF are significantly associated with diarrhoea in humans. We studied Apaches, primarily children, with diarrhoea attending an outpatient facility in Whiteriver, Arizona, from July 1986 through July, 1988. Stool cultures for isolation of ETBF and other diarrhoeal pathogens were taken from these persons as well as from age and time-matched control persons who did not have diarrhoea. ETBF were isolated significantly more often from persons with diarrhoea (12%) than from controls (6%), p = 0.03. Isolation was highest (20-24% of stool cultures positive) during the second and third years of life. The diarrhoeal syndrome associated with ETBF was non-specific, and most characteristic of a secretory, rather than inflammatory, type of diarrhoea. ETBF are significantly associated with acute diarrhoea in Apache children, and may be an important newly described cause of diarrhoea in humans.