Context: In recent years, the ketogenic diet has gained special relevance as a possible therapeutic alternative to some neurological and chronic diseases. Objective: The aim of this systematic review was to answer the following question: Does a ketogenic diet improve cognitive skills in patients with Alzheimer's disease, Parkinson's disease, refractory epilepsy, and type 1 glucose deficiency syndrome? To define the research question, the PICOS criteria were used, following the guidelines of the PRISMA method. Data sources: Medline/PubMed, Elsevier Science Direct, Dialnet, EBSCOhost, Mediagraphic, Sage Journals, ProQuest, and Wiley Online Library databases were used. Data extraction: After applying inclusion and exclusion criteria in accordance with the PRISMA method, a total of 63 entries published between 2004 and 2019 were used. Data analysis: The records extracted were analyzed from a qualitative approach, so no statistical analysis was carried out. Conclusion: Although scientific literature on the subject is scarce and there has tended to be a lack of scientific rigor, the studies reviewed confirmed the effectiveness of this diet in improving the cognitive symptomatology of the aforementioned diseases.