The clinical efficacy of combination therapy using Cefoxitin (CFX) and Amikacin (AMK) was studied in 19 patients with complicated urinary tract infections. Patients received 2 g of CFX i.v. and 100 mg of AMK i.m. twice a day. The overall clinical efficacy of treatment was evaluated by the criteria proposed by the UTI Committee, Japan, as excellent, moderate or poor. The overall clinical efficacy was excellent in 89%, moderate in 5% and poor in 5% of the patients. Of the 21 strains isolated from the patients, 20 strains (95%) were eradicated. No subjective side effects were observed. Drug-related aggravation in laboratory tests were observed slight elevations of glutomic-oxalacefic transaminase, glutamic-pyruvic transaminase and alkaliphosphatase in 2 cases, but all of them were minimal and reversible. Underlying condition-related aggravation was observed a slight elevation of BUN and creatinine clearance in 1 case. These results suggest that the combination therapy with CFX and AMK might be useful in the treatment of complicated urinary tract infections.