Background: The G2019S mutation is the most common pathogenic substitution in the leucine-rich repeat kinase 2 (LRRK2) gene, which has recently been identified in familial and sporadic Parkinson disease (PD). Objectives: To report the clinical characteristics of PD patients with homozygous LRRK2 6055G>A (G2019S) mutations and to compare them with previously published descriptions of heterozygous patients. Design: Descriptive clinical report from an international consortium of studies. Subjects: Patients with familial PD and homozygous LRRK2 mutations included 23 Tunisians, 2 Algerians, 2 US patients, 1 Canadian, and 1 Moroccan. Results: There were no observable differences between the homozygote and heterozygote phenotypes. Conclusions: Parkinson disease related to LRRK2 is characterized by typical clinical features, and the similarities between patients with homozygous and heterozygous mutations do not support a gene dosage effect. ©2006 American Medical Association. All rights reserved.