It really isn’t bad but some parts of it may be a little uncomfortable, and some people may be embarrassed when other people handle their private parts. Different doctors have their own preferences but this is basically what happens: First you will be asked to pee so your bladder will be empty when they start the test. They may collect a specimen for analysis at that time. Then they will prepare you for the test. That includes inserting a thin tube (catheter) through your penis into your bladder (this is the part that will be a little uncomfortable), another one into your rectum, a couple of patches with wires attached between your buttocks near your rectum, and maybe another patch on your thigh. The tubes are connected to a machine that measures pressure inside your bladder and also pumps water into it to fill it. The wires are also connected to the machine and the patches measure when and how hard you squeeze your sphincter muscles. Once everything is attached they will slowly fill your bladder with water and ask you to tell them when you feel that you can’t hold it any more. At that point they will let you pee and probably measure the flow rate while you pee. Don't worry if you don't tell them soon enough and have an accident. They expect that to happen and are prepared for it. The test measures how much liquid your bladder can hold, the pressure in your bladder when you really need to pee, and whether there are any spasms or sudden pressure increases that might cause an accident. The test is usually done lying on a table but can be done standing or sitting on a portable toilet.