Serum carbohydrate antigen 19-9 (CA 19-9), a marker of malignant tumors, is generally slightly elevated in benign conditions. We report a case of acute cholecystitis with a significantly elevated level of serum CA 19-9 based on positron emission tomography (PET)-computed tomography (CT) findings. A 65-year-old woman presented with abdominal pain and fever. A CT image revealed an enlarged gallbladder without tumor shadows. The C-reactive protein (CRP) level was elevated to 7.66 mg/dl. Moreover, the serum CA 19-9 level was significantly elevated to 19,392 U/ml. We started antibiotic treatment, because we suspected acute cholecystitis, but still, we could not ignore the possible presence of malignant tumors. After 11 days of antibiotic treatment, serum CRP and CA 19-9 levels decreased to 0.11 mg/dl and 1,049 U/ml, respectively. There was an accumulation of fluorine 18-labeled fluorodeoxyglucose (maximum standardized uptake value, 9.3) without tumor shadows in the liver, near the gallbladder, on the PET-CT examination. We considered the possibility that the inflammation had spread from the gallbladder to the liver, made a diagnosis of acute cholecystitis, and performed a cholecystectomy 33 days after treatment initiation. The serum CA 19-9 level decreased to 45 U/ml after the surgery. One year after the surgery, the patient was alive, and the serum CA 19-9 level was 34 U/ml. Acute cholecystitis with a significantly high elevation of the serum CA 19-9 level is rare. In such cases, it is important to confirm the change in the serum CA 19-9 level over time after antibiotic treatment and perform imaging studies to distinguish between inflammation and malignancy.