PURPOSE Streptozotocin-induced diabetic male rats underwent a pressure-flow study (P/Q study) under urethane anesthesia to determine whether partial cystectomy improves the percentage of bladder volume voided (V%) in a large bladder. MATERIAL AND METHODS P/Q studies were performed 12 weeks after the administration of streptozotocin to 8-week-old-rats. P/Q studies were also carried out after partial cystectomy in these diabetic rats. The method of the P/Q study was to perform cystometrograms via a cystostomy, and measure the micturition flow rate by differentiating the cumulative volume voided. RESULTS The measured urodynamic parameters show that bladder capacity increased and V% decreased significantly in diabetic rats compared with control rats. However, there were no significant differences in detrusor contraction strength (Wmax) as derived from the detrusor pressure and flow rate in the diabetic versus control rats throughout the experiment. Partial cystectomy in the diabetics improved V% dramatically from 53.4 to 73.7%. CONCLUSION These observations suggest that the increased post-void residual urine in diabetic rats is related to the increased bladder capacity rather than a decrease in detrusor contractility. Further, reduction cystoplasty increases the percentage of voided urine.