The most common etiology of acute pancreatitis (PA) in western Countries is gallstone disease. From recent data it has become clear that up to 30% of the so-called idiopathic AP are due to microlithiasis. Overlooking this diagnosis precludes to carry out a correct treatment and leads to relapses. In this prospective study bile for microscopic examination has been collected by duodenal intubation from 50 patients admitted because of AP of unknown etiology (history, laboratory data, ultrasound). In 21 patients the bile contained crystals, but only in 15 were considered as significant. Seven of these 15 patients underwent surgery which confirmed microlithiasis in all but one who had cholesterolosis. After cholecystectomy only one patient presented with a new episode of AP. In conclusion, microscopic examination of the bile may establish reliably a biliary origin in cases of AP of unknown etiology, allowing an adequate treatment of these patients and decreasing their risk of recurrences.