The role of genetics in Parkinson disease (PD) continues to be an area of considerable interest and controversy. We collected information involving the nuclear families of 948 consecutively ascertained PD index cases from the University of Virginia (UVA) Health System, the University of Medicine and Dentistry of New Jersey-Robert Wood Johnson (RWJ) School of Medicine, and Boston University (BU) School of Medicine. We performed a segregation analysis to assess evidence for the presence of a Mendelian pattern of familial transmission. The proportion of male (60.4%) and female (39.6%) cases, the mean age of onset (57.7 years), and the proportion of affected fathers (4.7%), mothers (6.6%), brothers (2.9%), and sisters (3.2%) were similar across the three sites. While most of the index cases were male, modestly more of the reported affected relatives were female. These analyses support the presence of a rare major Mendelian gene for PD in both the age-of-onset and susceptibility model. The age-of-onset model provides evidence for a gene that influences age-dependent penetrance of PD, influencing age of onset rather than susceptibility. We also found evidence for a Mendelian gene influencing susceptibility to the disease. It is not evident whether these two analyses are modeling the same gene or different genes with different effects on PD. The finding of significant genes influencing penetrance for PD raises the question of whether these may interact with environmental factors or other genes to increase the risk for PD. Such gene environment interactions, involving reduced penetrance in PD, may explain the low concordance rates among monozygotic twins for this disease. © 2002 Wiley-Liss, Inc.