The faecal outputs of 10 patients who complained of diarrhoea after ileal resection were measured whilst they consumed identical diets in which most of the fat was present as olive oil. No relationship was found between faecal weight and hydroxystearic acid production. In three patients the output of hydroxystearic acid was reduced by changing the dietary fat. There was no reduction in faecal weight. It is concluded that hydroxystearic acid does not contribute to diarrhoea in these cases.