Increased bladder mass occurs as a consequence of bladder outlet obstruction in animals and patients, and relief of bladder outlet obstruction reverses an increased bladder mass. Whether increased bladder mass is not only a consequence of bladder outlet obstruction but also a relevant risk factor for the progression of lower urinary tract symptoms associated with benign prostate hyperplasia cannot be decided due to a lack of appropriate data, most likely because bladder wall thickness is not routinely measured in clinical studies and/or routine clinical practice. Despite this lack of data, many urologists feel that increased bladder mass should be prevented or decreased to reduce the occurrence of serious complications.