Previously, chitinase 3-like 1 (CHI3L1) protein level was increased in various inflammatory conditions and cancers. This study was aimed to evaluate the plasma CHI3L1 level as a potential prognostic biomarker for Alzheimer's disease (AD). Forty-nine patients with mild cognitive impairment (MCI), 61 patients with mild to severe AD, and 35 healthy elderly controls were recruited for this study. They were given a comprehensive neuropsychological test battery including a mini-mental status examination (MMSE), clinical dementia rating (CDR), and neuropsychiatric inventory (NPI). The CHI3L1 levels were measured using a specific enzyme-linked immunosorbent assay in the plasma and cerebrospinal fluid (CSF). A significant increase in the mean plasma level of CHI3L1 was found in early AD patients compared to control subjects and MCI patients. No significant difference was found between MCI patients and controls. There was a significant positive correlation between CHI3L1 levels and neuropsychological test scores such as CDR and NPI in MCI and early AD patients. Our results demonstrate that CHI3L1 plasma levels are elevated in early AD compared to control or MCI patients. Thus, CHI3L1 plasma levels may be useful as a biomarker, reflecting disease severity in AD patients.