Clinical efficacy of piperacillin against 74 cases with complicated urinary tract infection was examined. Piperacillin was administered at the dose of 4 g (2 g twice daily) through intravenous drip infusion. The overall clinical value was rated in "excellent", in 9 cases, "moderate" in 34 cases and "poor" in 31 cases with a total efficacy of 58.1%. In the analysis of clinical values based on background, its efficacy was statistically significantly lower in the patients at a higher age, those with complication of diabetes mellitus, and those with indwelling catheter. In operated cases compared to non-operated cases, it was suggested to be more effective for improving the disturbances in urinary flow as a result of the removal of the underlying conditions by the operation. As to bacteriological efficacy, 64 out of 95 strains (67.4%) isolated were eradicated following its administration. Microbes which appeared after its dosing belonged to 9 classes of 18 strains, of which 5 strains (27.8%) of Serratia were identified. Side effects were 2.5% (3/119), no serious cases appeared. Changes in laboratory examination results were elevated GOT (2.5%), GPT (1.7%), and ALP (0.8%) values, all being transient hanges.