PURPOSE: It is essential to identify the presence of choledocholithiasis in patients with acute cholecystitis for elucidating the cause of cholecystitis and deciding the surgical planning. In this study, we tried to evaluate the incidence of choledocholithiasis and its ultrasonographic (US) diasgnostic values in patients with acute cholecystitis and to discuss its clinical significance. MATERIALS AND METHODS: During a 17-months period, seventy six patients with surgically proven acute cholecystitis who underwent preoperative US and endoscopic retrograde cholangiopancreatography (ERCP) were included in this study. US, ERCP and surgical findings of these patients were compared by reviewing their medical records as well as the US and ERCP images. The incidence of choledocholithiasis in these patients and its US diagnostic values were assessed. RESULTS: Of all 76 patients with acute cholecystitis, 20 (26%) were proved to have choledocholithiasis by ERCP and surgery. Among them, 13 were correctly diagnosed to have choledocholithiasis by US. The sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of US for the detection of choledocholithiasis in these patients were 65%, 100%, 100%, 89% and 91%, respectively. CONCLUSION: Our results indicated that one-fourth of patients with acute cholecystitis had choledocholithiasis, and the US diagnostic accuracy in detecting choledocholithiasis proved to be 91%, higher than previously reported results. Therefore, it is essential to undergo US in patients with acute cholecystitis to identify the presence of choledocholithiasis.