Alzheimer's disease (AD) is the most common cause of dementia. In 2015. it was estimated that 44 million people suffered from AD worldwide. Because of the aging population, a vast increase of disease incidence is expected throughout the next few decades. The disease is severe and incurable, and it can occur due to mutations in some genes (APP, PSEN1 and PSEN2), but most commonly occurs sporadically. It is estimated that the development of pathological processes in sporadic AD (also called AD with late onset) lasts in average around twenty years before the manifestation of the first clinical symptoms, and after the diagnosis it is difficult to influence the progression of the disease since there is already considerable damage to nerve cells throughout the brain. For the purpose of improving the efficiency of the available therapeutic procedures, it is necessary to diagnose the disease as early as possible. For this purpose, after eliminating possible reversible causes of dementia syndrome (pseudo dementia due to depression, hypothyroidism, vitamin B12 deficiency, etc.) in addition to neuropsychological testing we use neuroradiological diagnostics by MRI, CT and PET imaging and cerebrospinal fluid (CSF) biomarkers. ----- In this study we tried to evaluate the validity of determining the level of YKL-40 protein CSF as a potential new biomarker for early diagnosis of AD. The level of YKL-40 protein in the cerebrospinal fluid showed a moderate positive correlation with age and is significantly elevated in patients with AD compared to patients with mild cognitive impairment (MCI). P-tau181 as one of the biomarkers that make up the „gold standard“ is used in the research to validate the results and to compare the reliability of YKL-40 in the diagnosis of those MCI patients who will progress to AD. The study was conducted on 79 collected samples of CSF, and the subjects were divided into three groups: a control group (healthy subjects), a group of subjects with MCI and a group of patients with a clinical diagnosis of AD according to the criteria of the National Institute of Neurological and Communicative Disorders and Stroke – the Alzheimer's disease and Related Disorders Association (NINCDS-ADRDA) from 2011. The results showed that the concentrations of YKL-40 and p-tau181 positively correlated with each other. The concentrations of YKL-40 correlated with the age of subjects of all groups. The sensitivity of YKL-40 was 71.4%, and specificity was 66.7% while the sensitivity of p-tau181 was 44.6%, and specificity was 88.9%. According to these data YKL-40 and p-tau181 did not meet the criteria for the ideal biomarker, which determines that the sensitivity and specificity needs to be above 85%.