Osmolality, pH, and electrolyte concentrations in faecal fluid were measured in 23 patients referred to our department because of diarrhoeal disorders. The aim of the study was to ascertain whether such measurements could provide valuable diagnostic information in patients with diarrhoea. The patients were studied on a fat-restricted diet (70 g fat/day) and during fasting. Osmolality, pH, and concentrations of electrolytes in faecal water showed wide variations but were within normal ranges in most of the patients. The patients were grouped into secretory and osmotic diarrhoea on the basis of: 1) current assumptions on the pathogenesis of diarrhoea in different disorders; 2) persistence versus resolution of diarrhoea during fasting (resolution = decrease of stool mass to less than 200 g/24 h); and 3) an osmotic gap (measured osmolality -2 X (Na + K]. The accordance between these three ways of grouping was very incomplete. It is concluded that measurements of faecal fluid osmolality and electrolyte concentrations are of little value as diagnostic procedures in chronic diarrhoea. Determination of the osmotic gap and/or of the decrease of stool mass during fasting may help to elucidate the pathogenesis of diarrhoea in different disorders but does not seem diagnostically useful. Three patients turned out to be laxative abusers, and laxative ingestion should always be considered in chronic unsettled diarrhoea.