Clinical efficacy of combination therapy using Fortimicin (ASTM) and Cefotax (CTX) was studied in 63 patients with complicated urinary tract infections. The patients received 200 mg of ASTM i.m. and 1 g of CTX i.v. twice a day. The overall clinical efficacy of the treatment was evaluated by criteria proposed by the Japanese UTI Committee. The overall clinical efficacy rate was 68.3%. The rate was 67.3% in the patients with single bacterial infection and 72.7% in those with mixed infections. Subjective side effects were observed in one of the patients. Drug related aggravation in laboratory tests was observed in 8 items in 8 patients, but these were mild changes. We concluded that combination therapy with ASTM and CTX was useful for the treatment of complicated urinary tract infections.