© 2017 Elsevier Inc. Aims To explore whether type 2 diabetes (T2D) is a risk factor for dementia conversion in patients with mild cognitive impairment (MCI). Methods A longitudinal nested case–control study in which 101 T2D patients and 101 non-diabetic patients with MCI matched by age and gender were included. Results The dementia conversion rate was 57.4% in T2D patients vs. 42.6% in non-diabetic subjects (p = 0.02). T2D and APOE ε4 allele were independent risk factors for developing dementia. Conclusion T2D is an independent risk factor for dementia conversion in MCI patients. This finding has significant clinical implications.