Purpose: We evaluated the effect of transurethral resection of the prostate (TUR-P) in men with detrusor underactivity (DUA) who do not respond to conservative medical treatment. Materials and Methods: Of the patients who underwent TUR-P for LUTS at our institution, we reviewed the records of 71 patients who had undergone preoperative urodynamic study. According to the bladder outlet obstruction index and the bladder contractility index, the patients were divided into two groups: group A (25 patients) with unobstructed and underactive detrusor function, and group B (46 patients) with obstructed and/or normal detrusor contractility. We evaluated the difference of International Prostate Symptom Score (IPSS), quality of life (QoL), maximum flow rate (Qmax), post void residual (PVR) volume and the subjective satisfaction after TUR-P. Results: There were no significant differences preoperatively between the groups for age, Qmax and PVR. The total IPSS/QoL score was higher and the prostatic size was smaller in group A. After TUR-P in group A, the IPSS/QoL score and PVR were significantly improved; the Qmax was also improved, but this was not significant. There were significant improvements in all parameters in group B. Patients in group B showed the more significant improvement rate of the IPSS score and they were more satisfied after TUR-P than those in group A. Conclusions: Patients with DUA had less improvement in their symptom score and operative satisfaction than those with obstructed and/or normal detrusor contractility after TUR-P. However, there was significant improvement in IPSS/QoL and PVR after TUR-P in former group and 64% of these patients were satisfied. Therefore, TUR-P is thought to be an optional surgical procedure for treating the men with DUA who do not respond to conservative medical treatment.