Objective To investigate the correlation between chitinase protein(YKL-40)in cerebrospinal fluid(CSF)and longitudinal changes of cognitive function in patients with de novo Parkinson disease(PD).Methods PD group and healthy controls group were selected from the Parkinson Progression Markers Initiative(PPMI)database and their clinical data were collected at baseline,6 months,12 months,24 months,36 months and 60 months of follow-up.PD patients were divided into Parkinson disease normally cognitive(PD-NC),PD with mild cognitive impairment(PD-MCI),and Parkinson disease with dementia(PD-D)groups and comparison of clinical data between groups.We also investigated the correlation of baseline CSF YKL-40 concentration and rate of change in YKL-40 with longitudinal changes in other CSF biomarkers and clinical cognitive assessment results.Results A total of 195 patients were enrolled in PD group and 88 in healthy controls(HC)group.Baseline CSF YKL-40 levels in PD patients(119.08 pg/mL)were significantly lower than those in HC patients(132.64 pg/mL).The level of CSF YKL-40 in PD-MCI patients(127.51 pg/mL)was significantly higher than that in PD-NC patients(114.77 pg/ml)at baseline(P<0.001).In PD patients,baseline CSF YKL-40 was positively correlated with CSF Aβ42(β=0.526,P<0.001),T-tau(β=0.425,P<0.001),P-tau(β=0.373,P<0.001),and α-syn(β=0.525,P<0.001)levels,and was not correlated with clinical assessments of cognitive function.In mixed effect model analysis,we found that the higher the baseline CSF YKL-40 concentration in PD patients,the faster the increase of CSF T-tau(β=0.034,P=0.009)and P-tau(β=0.041,P=0.001)levels,and the faster the decrease of Hopkins verbal learning test(HVLT)delayed recall(β=-0.027,P=0.029)and HVLT retention(β=-0.030,P=0.022).We also found that higher rates of CSF YKL-40 change were associated with faster increases in CSF T-tau(β=0.045,P=0.001)and P-tau(β= 0.053,P<0.001)levels and faster declines in HVLT Delayed Recall(β=-0.026,P=0.049).Conclusion Our study suggests that CSF YKL-40 holds promise as a biomarker that can monitor the severity of cognitive decline in patients with new-onset PD.