To evaluate the relationship between lower urinary tract symptoms (LUTS), different diagnostic indicators of benign prostatic hyperplasia (BPH), and erectile dysfunction (ED) in a selected group of BPH patients with moderate-or-severe symptoms, for whom either transurethral or open prostatectomy was planned. Between 2003 and 2006, 453 patients were included in this study. LUTS and ED were assessed by validated symptom scales. Maximum and average urine flow rates and post-void residual urine volumes (PVRU) of all patients were measured. Additionally, prostate volumes for all patients were detected with transrectal ultrasound (TRUS). The incidence of LUTS and ED increased significantly with aging (P < 0.001). ED was reported to be 36% in men with moderate LUTS and 94% in men with severe LUTS (P < 0.001). The odds ratio for ED was 28.7 for severe LUTS. When age, IPSS, and IIEF scores were analyzed we observed that the occurrence of LUTS is an age-independent risk factor for the development of ED (P < 0.001). There was a positive correlation between IIEF score, Q (max) (r = 0.441, P < 0.001), and Q (ave) (r = 0.326, P < 0.001), and a negative correlation was found between IIEF score, prostate volume (r = -0.299, P < 0.001), and PVRU (r = -0.486, P < 0.001). The presence of LUTS, particularly severe LUTS, is an independent risk factor for ED. It is crucial to assess ED of the patient before BPH surgery, otherwise ED may be regarded as an outcome of the surgery rather than a preoperative, already existing condition.