A hydrocele is sac that is filled with fluid which forms on a testicle. It causes inflammation or swelling in the scrotum. They are most common in babies, but can affect males of any age. In adult men, it often affects those that are middle-aged or older. They are not a very serious condition as there are not any severe complications associated with the condition unless they are associated with an infection, tumor, or inguinal hernia.  They are also often painless. About 10 percent of males are born with a hydrocele.