The LRRK2 R1441G mutation was first identified in Basque families and it is responsible for 46% of familial Parkinson's disease (PD) and for 2.5% of sporadic PD in the PD population of Basque ascent. The aim of this study was to determine LRRK2 R1441G penetrance in PD in the Basque Country (Spain) to help in a more accurate genetic counseling. A total of 59 sibships containing 244 individuals, with a total of 40 PD-affected relatives, were studied. Genetic testing for the R1441G mutation in the LRRK2 gene was performed in 133 individuals and was positive in 51% of them. Lifetime penetrance of R1441G mutations turned out to be 12.5% at 65 years to 83.4% at 80 years. No gender differences were found in penetrance. © 2010 Movement Disorder Society.