Benign prostatic hyperplasia (BPH) is one of the most common problems faced by aging men and can be associated with bothersome lower urinary tract symptoms that affect quality of life by interfering with normal daily activities and sleep patterns. Despite the clinical importance of BPH, its pathogenesis is still poorly understood. Previously, our understanding of BPH was centered on bladder outlet obstruction being secondary to benign prostatic enlargement. However, prostate size itself is not correlated with the urine flow rate and symptomatology. The prostatic urethra is a bend tube and the increased prostatic urethral angulation shows a higher bladder neck on cystoscopic examination. Although some urologists suspected that the higher bladder neck might be a causal factor for BPH, the clinical significance of prostatic urethral angulation was previously underestimated. In this study, we propose a new hypothesis that prostatic urethral angulation is a causal factor for BPH. By applying the concept of fluid dynamics to the process of urination in the prostatic urethra, we show that the energy loss in this bending tube (the prostatic urethra) can occur during micturition and it increases proportionally to prostatic urethral angulation; this energy loss results in a decrease of the urine velocity and accordingly, the urine flow rate is inversely associated with prostatic urethral angulation. We also propose that BPH involves prostatic urethral angulation as well as the classical BPH triad of prostatic enlargement, bladder outlet obstruction, and symptomatology. Our hypothesis suggests that prostatic urethral angulation is an overlooked cause of bladder outlet obstruction and is a causal factor of BPH, and provides novel insight into the pathogenesis of BPH. Ultimately, the relationship between prostatic urethral angulation and urine flow rate, and other clinical factors including urodynamic parameters, the symptomatology, the response to treatment, and disease progression, need to be investigated in a clinical setting.