Cerebrospinal fluid (CSF) growth-associated protein 43 (GAP-43) is prominently elevated in Alzheimer's disease (AD) dementia patients in comparison to normal controls. CSF GAP-43 levels in mild cognitive impairment (MCI) individuals who have different clinical trajectories need to be studied.We examined 137 cognitively normal (CN) controls, 218 stable MCI patients (sMCI), 99 progressive MCI (pMCI) patients, and 120 AD dementia patients. Associations between the CSF GAP-43 levels and the four diagnosis groups were evaluated with multiple-variable linear regression. The relationships between CSF GAP-43 and core CSF biomarkers were assessed by Spearman correlations. Cox regression analysis was performed to assess the values of GAP-43 in predicting MCI conversion. We examined associations between baseline CSF GAP-43 levels and longitudinal cognitive function, hippocampal volumes, and brain glucose metabolism using linear mixed-effects models.CSF GAP-43 was elevated in the pMCI and AD groups in comparison to the CN group and in the pMCI and AD groups in comparison to the sMCI group. CSF GAP-43 significantly predicted conversion from MCI to AD. CSF GAP-43 was a significant predictor of cognitive decline, hippocampal atrophy, and brain hypometabolism over time. Furthermore, elevated CSF GAP-43 levels were associated with accelerated deterioration in cognition and neurodegeneration.CSF GAP-43 is increased in the predementia stage of AD, and it may enhance the neurodegenerative process. Future efforts on pharmacological interventions targeting synaptic dysfunction could be promising in AD treatment.© 2022. The Author(s), under exclusive licence to Springer Nature Switzerland AG.