Short-chain fatty acids (SCFA) in faeces were examined in 18 patients with the irritable bowel syndrome (IBS) during treatment with wheat bran or placebo. In the placebo period, the patients could be classified in accordance with the faecal concentrations of SCFA into one group with low concentrations (mean, 40 mmol/l; range, 19-77 mmol/l; 10 patients) and another with high concentrations (mean, 168 mmol/l; range, 145-187 mmol/l; 8 patients). The concentrations of SCFA differed (P < 0.001) in both groups from concentrations found in faeces from a reference group of nine normal individuals (mean, 114 mmol/l; range, 93-155 mmol/ I). Patients with low levels of SCFA had lower (P< 0.001) mean stool mass and longer (P < 0.05) transit times than those with high concentrations of SCFA in faeces. Ingestion of bran, although a precursor of SCFA, did not change faecal concentrations of SCFA. Abdominal pain, distension, and rumbling were not correlated to low or high concentrations of SCFA in faeces, nor did bran improve these symptoms when compared to placebo. The level of SCFA was rather constant intraindividually and independent of the variability of the daily faecal mass. It is concluded that patients with IBS apparently have continuously abnormal concentrations of SCFA in faeces, either high or low, which are unaffected by the treatment with bran and which hypothetically may be of pathophysiologic importance.