Alpha-blockers, the current common treatment for lower urinary tract symptoms (LUTS), are also used to treat bladder outlet obstruction (BOO), but the effect is not as clinically significant as in LUTS. All currently marketed phosphodiesterase type 5 (PDE5) inhibitors have recently been shown to significantly affect LUTS, although BOO-related efficacy has not been determined. Therefore, the extent of a causal relationship between LUTS and underlying benign prostatic enlargement (BPE) is questionable. LUTS may also be interpreted as symptoms related to detrusor overactivity, especially when no significant BOO is associated with BPE. Research is required to understand the efficacy of PDE5 inhibitors in LUTS but not in BOO. For vardenafil, nonclinical experiments and initial, preliminary clinical data suggest that the underlying effect may occur on the detrusor and not the prostate.