We report the case of a 62-year-old woman with Acinetobacter baumannii bacteremia. She was admitted to our hospital with ventricular tachycardia and was subsequently diagnosed with idiopathic ventricular tachycardia, with no structural heart disease. However, 12 days after admission, she suddenly developed a high-grade fever with chills and diarrhea. Her blood cultures revealed A. baumannii, and the patient was treated with meropenem and amikacin sulfate. Yet, the patient's symptoms and clinical signs became worse. We then began to administer a large quantity of intravenous ampicillin-sulbactam, and the patient improved dramatically. Although rare, bloodstream infection caused by A. baumannii tends to be severe. Therefore, when A. baumannii is found in a patient's bloodstream, clinicians should start appropriate treatment immediately and should recall ampicillin-sulbactam as a sensible option for treatment.