Acute cholecystitis consists of various morbid conditions, ranging from mild cases that are relieved by the oral administration of antimicrobial drugs or that resolve even without antimicrobials to severe cases complicated by biliary peritonitis. Microbial cultures should be performed by collecting bile at all available opportunities to identify both aerobic and anaerobic organisms. Empirically selected antimicrobials should be administered. Antimicrobial activity against potential causative organisms, the severity of the cholecystitis, the patient's past history of antimicrobial therapy, and local susceptibility patterns (antibiogram) must be taken into consideration in the choice of antimicrobial drugs. In mild cases which closely mimic biliary colic, the administration of nonsteroidal anti-inflammatory drugs (NSAIDs) is recommended to prevent the progression of inflammation (recommendation grade A). When causative organisms are identified, the antimicrobial drug should be changed for a narrower-spectrum antimicrobial agent on the basis of the species and their susceptibility testing results.