Basic and clinical research support a link between diabetes mellitus and Alzheimer disease (AD). However, the relationship with AD progression is unclear. This review focuses on the association between diabetes and cognitive decline in patients with AD. The literature published through May 2015 was searched in 3 databases: PubMed, Embase and Cochrane. Studies evaluating the effects of diabetes on patients with AD or cognitive decline were included, and extracted data were analyzed. A total of 10 articles met the inclusion criteria for review. The results of these studies were inconsistent in terms of the association between diabetes and cognitive decline. Only 2 studies demonstrated that the presence of diabetes was independently related to the progression of cognitive decline in the patients with AD, and 3 studies suggested that histories of diabetes were not correlated with the changes in cognitive function in patients with AD. Half of the included studies even indicated that histories of diabetes were associated with lesser declines in cognitive function in patients with AD. Current evidence indicates that the link between diabetes and cognitive decline in patients with AD is uncertain. Further clinical studies are needed, with larger samples, long-term follow up and an extended battery of cognitive assessments.