The efficacy, safety and usefulness were evaluated for a combined antibiotic tazobactam/piperacillin (TAZ/PIPC) consisting of a new beta-lactamase inhibitor, tazobactam (TAZ), and a broad spectrum penicillin antibiotic, piperacillin (PIPC), in chronic respiratory tract infections with PIPC as the control in a multi-institutional comparative study. The drugs used were a preparation containing 0.5 g of TAZ and 2.0 g of PIPC per vial (TAZ/PIPC group) and a preparation containing 2.0 g of (PIPC group). The drugs were intravenously injected one vial at a time twice a day for 14 days as a rule. The following results were obtained: 1. Clinical effect There was no significant difference between TAZ/PIPC (86% or 76/88) and PIPC (81% or 69/85). 2. Bacteriological effect There was no significant difference between TAZ/PIPC (93% or 42/45) and PIPC (88% or 36/41) in terms of bacterial eradication rates. In 34 patients with beta-lactamase-producing pyogenic bacteria, there was no significant difference between TAZ/PIPC (77% or 10/13) and PIPC (88% or 15/17). 3. Degrees of improvement in clinical symptoms, signs and laboratory findings The TAZ/PIPC group was likely to show reductions in fever and the amount of sputum soon after administration. 4. Side effects Incidences of side effects were 7% (7/96) in the TAZ/PIPC group and 3% (3/89) in the PIPC group, showing no significant difference between the two groups. The main symptoms were allergic reaction and gastrointestinal symptoms. 5. Abnormal clinical laboratory test values The incidence was 17% (15/89) in the TAZ/PIPC group and 21% (18/87) in the PIPC group. The main symptoms were eosinophilia and hepatic dysfunction, and most of these symptoms were mild. 6. Usefulness The usefulness rates in the TAZ/PIPC group were 80% (71/89) and 78% (66/85) in the PIPC group, showing no significant difference. Thus, TAZ/PIPC exhibited excellent clinical effects and presented no troubles with safety. When comprehensively evaluated, TAZ/PIPC appears to be a very useful drug for the treatment of chronic respiratory tract infections.