Epidemiological and basic science evidence suggests a possible shared pathophysiology between type 2 diabetes mellitus (T2DM) and Alzheimer's dementia (AD). Aim: To compare between type 2 diabetic, non-diabetic AD and control regarding cognitive functions and metabolic abnormalities, assess cerebral blood flow (CBF) and correlate it with the severity of both DM and AD. Patients and methods: Three groups were included : Group (Gr) I: 20 patients with AD + DM, Gr II: 26 patients with AD only and were matched with Gr III including 20 normal control (NC). The following were done for all patients: A battery of neuropsychological tests, laboratory tests (lipid profile, serum uric acid, fasting blood glucose, post prandial blood glucose, glycosated hemoglobin, insulin level, insulin resistance and serum insulin like growth factor-1) then assessment of CBF using extracranial carotid duplex (ECD) and single photon emission computerized tomography (SPECT). Results: A statistically significant difference between Gr I and Gr II in concentration subtest of Addenbrooke's cognitive examination revised being better in Gr I, A statistically significant difference between Gr I and Gr II regarding insulin level and insulin resistance being higher in Gr 1. No significant difference was detected between Gr I and Gr II regarding cerebral blood flow volume (CBFV). Duration of AD showed highly statistically significant negative correlations with CBFV Gr I and II. DM duration showed statistically significant negative correlations with total CBF uptake in left cingulate (p=0.047) and right cerebral hemisphere (p=0.01) in Gr I. Conclusion: Insulin and insulin resistance play an important role in the development and progression of AD and DM. Hyerinsulimemia and insulin resistance have great impact on cognitive function and cerebral blood flow. Our results favor "vascular hypothesis" of AD.