Background: Several pathogenic mutations have been reported in the leucine-rich repeat kinase 2 gene (LRRK2) that cause parkinsonism. The "common" LRRK2 G2019S kinase domain substitution has been reported to account for approximately 5% of familial and 1% of sporadic Parkinson disease. Objective: To observe the clinical heterogeneity presented by LRRK2 kinase mutation carriers. Design, Setting, and Participants: We screened 130 patients with pathologically confirmed Parkinson disease and 85 controls for 3 LRRK2 kinase domain pathogenic substitutions: I2012T, G2019S, and I2020T. Main Outcome Measures: Detailed clinical phenotypes for individuals who screened positive for LRRK2 mutations. Results: Five LRRK2 G2019S carriers were identified, of whom 4 had Parkinson disease (clinically and pathologically confirmed), and the fifth was a control subject who died at age 68 years after an acute myocardial infarction with no evidence of neurodegenerative abnormalities. There was no evidence of the I2012T or I2020T mutation in these participants. Conclusions: The underlying disease mechanisms of LRRK2 G2019S-associated parkinsonism are similar to those of typical Parkinson disease. The identification of a control subject raises important questions concerning genetic diagnosis and counseling.