The most common cause of a lump that disappears on the back of the wrist is a ganglion cyst. But it could be a tumor, bone spur, or even an infection. It's always best to have a doctor check to make sure it's not something more serious.
Sometimes, wrist ganglion cysts go away. In many cases they start to get bigger, not smaller. If it gets big enough, it can put pressure on the nearby tendons, ligaments, blood vessels, and nerves. The result may be pain, loss of motion, numbness and tingling, and swelling.
The best treatment is removal of the entire cyst. This includes the fluid inside, the outside covering, and the place where it attaches to the joint capsule called the stalk.