The contribution of various factors to diarrhoea in coeliac disease (CD) was evaluated by measuring jejunal villous height, faecal masses, faecal fat, and faecal bile acids in 8 healthy subjects and in 37 consecutive patients before and during a gluten-free diet (GFD) and while receiving cholestyramine. Jejunal villous height was inversely correlated with faecal fat but not with faecal mass or faecal bile acids. A strong positive correlation was found between faecal fat and faecal mass and between the respective changes caused by GFD in these variables, whereas the correlations between faecal bile acids and faecal volume or between these changes caused by GFD were weak or nonsignificant. The cholestyramine treatment enhanced faecal bile acid and fat excretions and increased faecal volume without a significant effect on faecal solids or on the frequency of bowel movements, which in turn was significantly reduced by GFD. Thus, the present results show that the impaired fat absorption owing to mucosal damage of the upper small intestine markedly contributes to faecal volume and the frequency of bowel movements in CD and may be an important factor responsible for diarrhoea in coeliac patients, whereas no evidence was obtained for bile acid diarrhoea in CD.