Background Mutations in Leucine-rich repeat kinase 2 (LRRK2) enhance levels of autophosphorylated LRRK2 protein and are the most common known cause of inherited Parkinson disease (PD). LRRK2 has been further implicated in susceptibility to idiopathic PD in genetic association studies. Objective To compare autophosphorylated Ser(P)-1292 LRRK2 levels from biobanked urine samples with clinical data in PD and controls. Methods Ser(P)-1292 LRRK2 levels were measured from urine exosome fractions from 79 PD and 79 neurologically healthy controls enrolled into the Parkinson Disease Biomarker Program at the University of Alabama at Birmingham. Results Ser(P)-1292 LRRK2 levels were higher in males than females (p<0.0001) and elevated in PD compared to controls (p=0.0014). Ser(P)-1292 LRRK2 levels were higher in PD cases with worse cognition and correlated with poor performance in MoCA (r=−0.2679 [−0.4628, −0.0482]), MDS-UPDRS subscales 1 and 2 (r=0.2239 [0.0014, 0,4252], 0.3404 [0.1276, 0.5233] respectively), Epworth Sleepiness Scale (r=0.3215 [0.1066, 0.5077]), and Modified Schwab and England Activities of Daily Living Scales (r=−0.4455 [−0.6078, −0.2475]). Ser(P)-1292 LRRK2 levels predicted those with worse cognitive impairment in PD patients with some success (c=0.73). Conclusions Urinary exosome Ser(P)-1292 LRRK2 levels are elevated in idiopathic PD and correlated with the severity of cognitive impairment and difficultly in accomplishing activities of daily living. These results implicate biochemical changes in LRRK2 in idiopathic PD.