A Hydrocoele occurs when there is a build-up of fluid in the sac that surrounds the testicle. A small amount of fluid in this sac is normal, but the excess fluid results in a painless swelling of the scrotum.
In infants, a hydrocoele usually occurs because an opening between the abdomen and the scrotum hasn't closed off during development. If it persists for longer than a year after birth, then surgical repair of this opening is usually required. This is called a ligation of the patent processes vaginalis.