Alzheimer's disease (AD) is the main cause of dementia worldwide, which implies an important socioeconomic problem in developed countries. Efforts to find biomarkers to diagnose AD have been intensified, especially, to detect cognitive impairment in its early stages, also known as mild cognitive impairment (MCI). Besides, there are individuals referring memory loss that is unnoticeable in the family environment and presenting normal neuropsychological tests. The former patients are included in a clinical picture that has been recently called subjective memory complaints (SMC). To achieve an early diagnosis, optical coherence tomography (OCT) has been used to measure macular thickness in patients diagnosed with MCI (supported by neuropsychological tests) and SMC (not based on neuropsychological battery). Statistically significant differences have been found in the macular thickness of the control group (274.96 ± 17.61 m) and for both MCI (259.48 ± 22.39 m) and SMC (261.45 ± 24.26 m) patients. In the near future, OCT could become a reliable biomarker and a useful tool for AD screening as well as for the monitoring of the cognitive impairment associated with AD.