Purpose: To investigate the possibility of improving the diagnostic accuracy by identifying the time course of various Ultrasonography (US) findings of acute cholecystitis. Subjects and Methods: The subjects were 69 patients (41 male and 28 female patients, mean age: 54.0 years old) who developed acute cholecystitis between January 2010 and February 2015, in whom the onset date and time could be identified and active treatment was not performed prior to US examination. We investigated the time-course incidences of various US findings (gallbladder enlargement, biliary sludge, wall thickening, intramural hypoechoic layer, fluid retention around the gallbladder, and abscess formation). Results and Discussion: The incidences of swelling, biliary sludge, gallbladder wall thickening, hypoechoic layer, fluid retention around the gallbladder, and abscess formation increased over time in this order. Swelling alone was noted in 70.0% (7/10) of patients who were examined within 10 hours after onset, and all such patients were examined by US within 15 hours after onset, suggesting that acute cholecystitis cannot be ruled out within 15 hours, even though only swelling is observed. In 91.7% (33/36) of patients who were examined by US 24 or more hours after development, at least swelling + biliary sludge + wall thickening were observed, suggesting that when swelling, biliary sludge, and wall thickening are not observed 24 or more hours after development, acute cholecystitis is unlikely. Conclusion: Diagnostic accuracy may be improved by investigating the order of the development of various US findings of acute cholecystitis and the time course of their development.