Background and Objectives: Mild Cognitive Impairments (MCIs) are identifiable clinical entities, in neurodegenerative forms, as prodromal for Alzheimers type (DAT) or in vascular forms, as prodromal for vascular dementia (VaD). The present longitudinal study compares and contrasts MRI abnormalities among MCI subjects as they progress to DAT versus VaD. Subjects converting to DAT and VaD confirmed ultimate diagnosis during MCI staging. In "mixed cases" the predominant MRI pathology was judged the primary cause. Subjects and Methods: Subjects (n=153) were selected from elderly outpatient volunteers who have been enrolled for 25 years in planned longitudinal studies of aging, stroke and dementia. Cognitively normal (CN, n=52), MCI of neurodegenerative (N-MCI, n=30) and vascular (V-MCI),n=35) subtypes, plus converted DAT (n=19) and VaD (n=17) were diagnosed according to established protocols. Combined Mini-Mental-Cognitive Capacity Screening Examinations (CMC) screened, identified and confirmed MCIs or dementias. Cerebral MRI abnormalities were analyzed utilizing volumetric measurements and visual rating scales. Results: Compared with persistently cognitively normal subjects, MCI subjects and converted dementias were significantly older without significant gender differences, but cognitively impaired subjects were older than the CN group since age is a risk factor for cognitive decline. Histories of hypertension, heart disease, diabetes mellitus, TIAs and strokes were more frequent among subjects with VMCI and VaD, confirming that all vascular risk factors contribute to vascular cognitive decline, but since vascular risk factors were treated, not all progressed to VAD. Family history of neurodegenerative disease, particularly DAT, were more prevalent among NMCI and converted DAT subjects. VMCI showed more extensive leucoaraiosis and lacunar infarcts than subjects with NMCI. NMCI, prodromal for dementia of Alzheimers type (DAT), showed more medial temporal lobe atrophy with enlarged temporal horns, and fewer vascular lesions.