Up to 30% of patients with acute pancreatitis are diagnosed of idiopathic acute pancreatitis after an initial evaluation including a complete clinical history, physical examination, analysis with calcium and triglycerides determination, and at least one transabdominal ultrasonography. Unexplained pancreatitis represents a diagnostic challenge, although after different explorations a cause is found in the majority of these patients. During the last years endosonography has proved to be a low morbidity exploration very useful in the evaluation of patients with this entity. In this article we review the role of endosonography in the etiologic study of patients with idiopathic acute pancreatitis.