The authors performed a prospective, unbiased analysis of a cohort of young patients assessed consecutively with the question of dementia. The onset of patients' cognitive symptoms was prior to the age of 65 years. A study group of 226 patients was followed for a mean duration of 4.59 +/- 2.23 years (1 SD; range, 0.04-7.86 years). The diagnoses were established using published diagnostic criteria. A diagnosis of dementia was made in 112 patients (49.56%). Psychiatric disease was the most common diagnosis in those who did not have dementia (24.3%) followed by frontotemporal lobar degeneration (19.0%), Alzheimer's disease (11.9%), patients with cognitive symptoms who obtained normal neuropsychometric profiles (10.6%), nonneurological disorders (eg, obstructive sleep apnea [8.4%]), neurological disorders (eg, Parkinson's disease [4.9%]), and mild cognitive impairment (4.9%). The frequencies of frontotemporal lobar degeneration and psychiatric disease were higher than Alzheimer's disease, unlike in older populations.