Dementia is the development of multiple cognitive deficits that includes memory impairment and at least one of the following--Aphasia, apraxia, agnosia or disturbances in executive functioning. The common causes of dementia among the elderly are Alzheimer's disease, vascular dementia, mixed dementia and Lewy body disease. The concept of reversible dementia was introduced in 1980 when a task force sponsored by National Institute of Ageing found 10-12% of dementia cases in older group to have reversible causes such as metabolic-nutritional, drugs, infections, psychiatric disorders etc. In our series of 76 patients in the presenile age group (<65 years), 34.21% (26/76) had a reversible condition underlying the dementia. 43.42% (33/76) had vascular dementia, 13.15% (10/76) had Alzheimer's disease and 9.21% (7/76) had mixed dementia. Hypertension, hyperlipidemia and diabetes mellitus were commoner in the vascular dementia group as compared to the Alzheimer's group. Evaluation of MRI as a tool in diagnosis of dementia showed increased sensitivity of MRI towards detecting lacunes. The potentially reversible dementias comprised infections 14.47% (11/76), metabolic-nutritional 14.47% (11/76) and autoimmune diseases 3.94% (3/76). These were characterized by a subcortical dementia. Four month follow up of MMSE in this group showed significant and sustained improvement in the metabolic nutritional group.