The causes of abdominal pain in patients with AIDS are different from those of the general population, and there are no guidelines as to which investigations are optimal. We reviewed our experience of 63 patients who presented with abdominal pain as the main reason for admission to the AIDS unit at St Stephen's and Westminster Hospital between January 1988 and January 1990. All patients were assessed within the same structured diagnostic programme. Thirty-five had upper abdominal pain, predominantly in the right upper quadrant in 27; seven had lower abdominal pain, which was concentrated in the right iliac fossa in three; 21 had diffuse abdominal pain. The causes of pain were determined satisfactorily in the majority of patients using routine gastroenterological investigations. The predominant site of pain had considerable predictive value in the diagnosis. The commonest cause of right upper quadrant pain was sclerosing cholangitis (17 of 27); endoscopic retrograde cholangio-pancreatography was necessary for a confident diagnosis. Diarrhoea was frequently associated (15 of 21) with diffuse abdominal pain; the commonest cause (six of 21) was cytomegalovirus colitis. Neoplasia was the cause of abdominal pain in eight patients. The other causes of abdominal pain and the utility of various investigations are discussed. An investigatory route which may provide maximum information has been suggested.