We measured the concentration of bile acids in gastric aspirates from patients who had had operations for peptic ulcer. Some patients were asymptomatic and some had postoperative symptoms of the type that have been attributed to duodenogastric reflux. Samples were obtained via a nasogastric tube when the patients were fasting, after food, after pentagastrin, and overnight. We related the concentration and amount of bile acid and the volume aspirated to the presence or absence of symptoms and compared the results with radiological and endoscopic assessments of duodenogastric reflux. The most useful index to discriminate between symptomatic and asymptomatic patients was the amount of bile reflux in half an hour's aspiration from the fasting stomach; this we have termed 'fasting bile reflux' (FBR) and expressed as mumol bile acids refluxing/hour. A figure greater than 120 mumol/h was present in 17 of 22 symptomatic patients and in all who complained of bile regurgitation or bile vomiting. The FBR was less than 120 mumol/h in all of 20 asymptomatic patients, although some of them had reflux detected radiologically and endoscopically.