Arbekacin sulfate (ABK) was administered by intravenous drip to pneumonia patients infected with methicillin-resistant Staphylococcus aureus (MRSA), and the efficacy and the safety were objectively evaluated by the executive committee. The daily dose was determined in principle as 150-200 mg, two times a day, 30-90 minutes drip infusion, and the dose was to be changed at each special occasion. Combined therapy with other antibiotics was scheduled in severe cases at a decision of the physician in charge. Data of 18 cases were accumulated. The efficacy could be evaluated for 12 cases (4 cases with ABK alone, and 8 cases with combined therapy), and the safety was evaluated for 18 cases. The clinical efficacy was: excellent, 1; good, 4; fair, 5; and poor, 2. The efficacy rate was 41.7%. The bacteriological effect was: eradicated, 2 (16.7%); decreased, 2; and no change, 8. There found no side effects except 3 cases of abnormal laboratory data, two abnormal renal functions(11.1%) and one abnormal hepatic function (5.5%). In one of the renal disorders, decreased dose of ABK improved the function. In the other case, the impaired renal function lasted until death by heart failure. In the case of abnormal function, discontinuing the ABK therapy improved the hepatic function. In the 4 out of 5 cases that showed excellent or good clinical efficacy, patients recovered within relatively early days of ABK therapy. The average days for recovery was 7.8.