According to a 2004 article by Jordana Lewis and Jerry Adler of Newsweek, research on being unforgiving shows that holding a grudge can lead to "increased blood pressure and hormonal changes -- linked to cardiovascular disease, immune suppression and, possibly, impaired neurological function and memory."
Expert Quotes About Being Unforgiving:
Dr. Dean Ornish: "In a way, the most selfish thing you can do for yourself is to forgive other people ... When I talk about forgiveness, I mean legging go, not excusing the other person or reconciling with them or conding the behavior. Just letting go of your own suffering."
Everett Worthington: "It happens down the line, but every time you feel unforgiveness, you are more likely to develop a health problem."
Dr. Edward M. Hallowell: "It's a process, not a moment ... If you devote your life to seeking revenge, first dig two graves."
Source for quotes: Jordana Lewis and Jerry Adler.