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Mitochondrial DNA regulates mitochondrial function which is altered in both idiopathic and familial forms of Parkinson's disease. To investigate whether these two disease forms exhibit an altered regulation of mitochondrial DNA we measured cell free mitochondrial DNA content in cerebrospinal fluid (CSF) from idiopathic and LRRK2-related Parkinson's disease patients. The concentration of mitochondrial DNA was measured using a digital droplet polymerase chain reaction technique in a total of 98 CSF samples from a cohort of subjects including: 20 LRRK2G2019S mutation carriers with Parkinson's disease, 26 asymptomatic LRRK2G2019S mutation carriers, 31 patients with idiopathic Parkinson's disease and 21 first-degree relatives of LRRK2 Parkinson's disease patients without the mutation. Here we report that LRRK2G2019S mutation carriers with Parkinson's disease exhibit a high concentration of mitochondrial DNA in CSF compared with asymptomatic LRRK2G2019S mutation carriers and with idiopathic Parkinson's disease patients. In addition, idiopathic, but not LRRK2 Parkinson's disease is associated with low CSF concentration of α-synuclein. These results show that high mitochondrial DNA content in CSF distinguishes idiopathic from LRRK2-related Parkinson's disease suggesting that different biochemical pathways underlie neurodegeneration in these two disorders.