INTRODUCTION: Alzheimer's disease and vascular dementia are the two most frequent forms of severe cognitive impairment. Their distinction is of undisputed prognostic and therapeutic importance. OBJECTIVE: To determine whether differences exist between cerebral vasoreactivity parameters measured by transcranial Doppler in patients with Alzheimer's disease and probable vascular dementia clinically defined. METHODS: We recruited 70 patients divided into two groups: Alzheimer's and probable vascular dementia clinically defined. We recorded clinical data and rated the severity of dementia. Subsequently we performed a transcranial Doppler consigning hemodynamic parameters before and after the apnea test. RESULTS: The cerebral vasoreactivity measured by transcranial Doppler showed a sensitivity of 52% (95% CI: 33.5-69.7) and specificity of 80% (95% CI: 66.18-89.1) for the differential diagnosis of probable vascular dementia. There was not a relationship between the severity, time of illness or the presence of vascular risk factors with cerebral vasoreactivity. CONCLUSION: This method can be a diagnostic aid test for detection of probable vascular dementia. We recommend further studies to establish relationships with respect to cerebral vasoreactivity, vascular risk factors, disease duration and severity in dementia.