OBJECTIVE To look for the presence of bile acid malabsorption in HIV infected patients with chronic diarrhoea and determine whether bile sequestering agents may have a role in palliating this common problem. METHODS Nineteen HIV infected patients with chronic diarrhoea (duration > one month) poorly controlled on conventional treatment were investigated using the seven day retention of 75seleno-23-homocholic acid taurine (SeHCAT) as a measure of bile acid loss from the enterohepatic circulation. Patients with evidence of bile acid malabsorption were offered cholestyramine. RESULTS Sixteen (84%) had evidence of bile acid malabsorption (< 15% retention at seven days). Ten of the 16 patients with bile acid malabsorption had terminal ileal biopsies-six had ileitis and four normal histology, suggesting that malabsorption is not always related to terminal ileitis. Thirteen patients with bile acid malabsorption have been treated with cholestyramine and 11 have reported a symptomatic response. CONCLUSIONS Bile acid malabsorption can be demonstrated in some cases of HIV associated chronic diarrhoea and we suggest a therapeutic trial of a bile sequestering agent in patients whose symptoms are not well controlled using conventional anti-diarrhoeal agents.