Parkinson's disease (PD) is a growing health problem in the elderly, and genetic factors may provide substantial clues toward eventual treatment and/or prevention of the disorder. The National Academy of Sciences/National Resource Council (NAS/NRC) World War II (WWII) Veteran Twins Cohort Registry is one of the few population-based groups of elderly twins. Previous studies in this group indicated that genetic factors were involved only in early-onset PD, however there was no follow-up to these studies.