Background/Aims: EUS can detect bile duct stones (BDS) that are undetectable on multidetector computed tomography (MDCT). BDS associated with acute biliary pancreatitis (ABP) are small and tend to be excreted spontaneously. This study evaluated the usefulness of early EUS in patients with ABP and undetectable BDS on MDCT. Methods: Forty-one patients with ABP and undetectable BDS on MDCT underwent EUS within 24 hours of admission and were diagnosed with BDS, sludge, dilated common bile duct (CBD), or normal CBD. ERCP was performed in patients with BDS, sludge, or clinical deterioration. The diagnostic yield and the effects of early EUS on morbidity, mortality, and the length of hospitalization were evaluated. Results: EUS detected BDS or sludge in 48.8% of patients examined. BDS was the diagnosis in 13 patients, sludge in seven, and neither for 21 patients. ERCP was performed in 20 patients with BDS or sludge, in two patients with coexisting cholangitis, and in one patient with worsening liver function tests. ERCP identified BDS in 12 patients and sludge in seven. No lesions were diagnosed in four patients by ERCP. All patients improved, and the length of hospitalization in patients with ERCP was 9.0 days, without ERCP 7.1 days. Two patients with major complications by ERCP were hospitalized for a prolonged time. Conclusions: Early EUS may be useful to select patients for therapeutic ERCP in cases of suspected ABP with undetectable BDS on MDCT. (Korean J Gastroenterol 2016;68:202-209)