Four methods of gall stone diagnosis after an attack of acute pancreatitis are analysed. Of 128 consecutive patients with acute pancreatitis, 99 patients were discharged from hospital without a definite aetiology. These patients had biochemical tests performed on admission and ultrasonography and oral cholecystography performed six weeks later. The sensitivity for ultrasonography was 87% and the specificity was 93%; the respective figures for oral cholecystography were 83% and 90%. The predictive value of positive ultrasonography was 100% and of negative ultrasonography 75%; the respective values for oral cholecystography were 95% and 68%. A combination of ultrasonography and oral cholecystography failed to detect nine of 70 patients with gall stones (13%). Of 35 patients with normal ultrasonography and oral cholecystography, 33 patients had an endoscopic retrograde cholangiogram (ERCP) which showed gall stones in a further seven patients. All three methods failed to reveal gall stones in two patients, confirmed by laparotomy. The sensitivity of admission biochemical analysis was 73% and the specificity was 94%; the predictive value of a positive result was 97% and of a negative result was 57%. Biochemical analysis predicted gall stones in six of the seven patients shown by ERCP. Only 9% of patients were finally considered to be idiopathic. In conclusion ultrasonography is the investigation of choice and ERCP should be undertaken in all patients who have normal ultrasonography and/or oral cholecystography but have biochemical criteria indicative of gall stones.