Purpose: To create a prediction model that could preoperatively measure the success of the transurethral prostate resection (TURP) because of bladder outlet obstruction. Methods: Patients aged 50–80 years applied with TURP were examined prospectively and evaluated in respect of the preoperative and postoperative values of maximum flow rate (Qmax), international prostate symptom score (IPSS), quality of life (QoL) score and post-voiding residual (PVR) urine amount. On the preoperative transabdominal ultrasonography, total prostate volume (TPV), and protruding prostate lobe volume (PPLV) were measured and the protruding ratio (PR), as the ratio of PPLV to TPV, was calculated. Based on the mean of the preoperative and postoperative Qmax difference (Qmax-D) value, Group 1 (n = 33) was defined as “low efficacy” and Group 2 (n = 30) as “high efficacy”. Results: A correlation was determined between the QMax-D, IPSS difference, PVR difference, and QoL difference measured for the efficacy of the operation, and TPV, PPLV and PR. The results of multivariate analysis showed the main effect to be created by PR (p = 0.000; OR 1.596). In the evaluation with receiver operating characteristic curve analysis of high efficacy obtained in the TURP, a significantly powerful effect of the measurements of PPLV area under curve (AUC 0.922 [0.855–0.989] p= 0.000) and PR (AUC 0.954 [0.982–1.000] p = 0.000) was determined. The cutoff value of 11.5 was detected for PR. Efficacy sensitivity and the positive predictive values were recorded as 93.3%, and specificity and negative predictive value as 93.9%. Conclusion: When determining candidate patients for TURP surgery, measurements of the PPLV and especially the PR should be taken into consideration in the preoperative prediction of efficacy.