At age 18, if one of your parents has HD, you are eligible to be genetically-tested for the mutant gene that causes Huntington's disease. Your chance of having the gene is the equivalent of the flip of a coin, and if you have the gene, you will develop HD. It's not an "if," it's a "when."
The discovery of the genetic mutation for HD in 1993 made genetic screening possible, and so every generation within an HD family "at risk" is burdened with an incredibly complex question: Do you want to know if you will get HD?