In the authors' experience gallstones, particularly when minute, constitute the most frequent cause of acute pancreatitis. The size of the calculi (about one millimeter) explains the two major characteristics of microlithiasis: it is "illegible" and therefore difficult to detect and frequently undiagnosed; it is "migratory", like urinary lithiasis, and may be responsible for transient hyperpressure in the pancreatic ducts resulting in acute pancreatitis. In all cases of acute pancreatitis, the possibility of biliary microlithiasis should be thoroughly investigated and relapses should be prevented by cholecystectomy with clearing of the biliary ducts under radiosurgical and/or endoscopic control.