Objective:To evaluate the clinical application value of endoscopic ultrasonography (EUS) in the etiological diagnosis of patients initially diagnosed with idiopathic acute pancreatitis (IAP).Methods:Clinical data of 128 patients who underwent further EUS and magnetic resonance cholangiopancreatography (MRCP) after initial diagnosis of IAP at the Gastrointestinal Endoscopy Center of the First Affiliated Hospital of Naval Medical University between January 2015 and February 2022 were collected and divided into a single-episode group (single-episode group, 51 cases) and a multiple-episode group (recurrent group, 77 cases) based on the number of AP episodes. The data and the diagnosis of the etiology of IAP in the two groups by EUS were analyzed and compared with the etiological diagnosis results of MRCP.Results:The differences on basic information such as gender, age, history of smoking, history of alcohol consumption, family history of pancreatic disease, history of cholecystectomy, abnormality of liver function, and severity of pancreatitis between the single-episode group and recurrent group of IAP patients were not statistically significant. The etiology was clarified in 79 (62%) IAP patients after EUS examination, of which 55 (43%) cases had biliary disease (gallstones, microlithiasis, biliary sludge) and 24 (19%) cases had pancreatic disease (chronic pancreatitis, pancreatic divisum, pancreatic interstitial or pancreatic ductal changes). The percentage of patients with biliary disease as the cause of IAP was significantly higher in the single-episode group than in the recurrent group (59% vs 32%), while the percentage of patients with pancreatic disease as the cause of IAP was higher in the recurrent group than in the single-episode group (25% vs 10%), with statistically significant differences ( P values=0.004 and 0.035, respectively). The performance of EUS in diagnosing the etiology of IAP was significantly higher than that of MRCP (62% vs 19%, P=0.032), where EUS was more accurate in detecting biliary microlithiasis or biliary sludge (43% vs 9%, P<0.01). EUS was also superior to MRCP in identifying subtle changes in chronic pancreatitis lesions (small pancreatic nodules, patchy hyperechogenicity, etc.) and intraductal papillary mucinous neoplasms(17% vs 7%, P<0.05), but was inferior to MRCP in identifying pancreatic divisum (2 cases vs 4 cases). Conclusions:In view of high diagnostic accuracy and safety of EUS in diagnosing biliary diseases, and based on the fact that most IAPs in China are due to biliary diseases, EUS based management strategy can be considered to be a reasonable approach for evaluation of IAP patients. The MRCP can be used as a supplement to the EUS to identify a controversial etiology.