Invasive urodynamics involves putting a tube (catheter) into the bladder via the penis, and another into the rectum, to measure bladder and abdominal pressures while the bladder is filled with a sterile fluid. Invasive urodynamics can measure bladder pressures during filling and voiding, and bladder outlet obstruction pressures. The procedure is considered safe, but some men find it uncomfortable or undignified, and a few develop urine infection afterwards. We think that it could be useful to select the men who should and should not have surgery. However, no studies have been conducted so far to tell us if this is true: UPSTREAM is designed to find out if the invasive tests are worthwhile.