A prospective study of the accuracy of real time ultrasonography in the detection of gallstones was undertaken in 180 patients from February 1987 to February 1988. The ultrasound findings were compared with single dose oral cholecystography (OCG), and with the surgical findings where surgery was undertaken. Ultrasonography gave more accurate results than OCG, with an overall accuracy in the surgically proven patients of 97.1%, no false positive findings and a 2.9% false negative rate. OCG gave an accuracy of 80% with no false positive findings and a 20% false negative rate. Ultrasound was particularly valuable where there was non visualisation of the gall bladder at OCG, giving an overall accuracy of 93.3% in such patients. Ultrasonography is a non invasive, simple, safe and economic diagnostic test of high accuracy in the diagnosis of cholelithiasis and of particular benefit in those patients unsuited for OCG.