Objectives: To examine the incidence of dementia in subjects with mild cognitive impairment (MCI) in the Cardiovascular Health Study Cognition Study. Design: Prospective epidemiological study. Setting: The Cardiovascular Health Study Cognition Study of Pittsburgh, Pa, was conducted from 2002 through 2003 to determine the incidence of dementia in participants classified as having MCI in 1998 and 1999. Subjects: There were 136 subjects with MCI. Mild cognitive impairment was subclassified as MCI amnestic type and MCI multiple cognitive deficits type (MCI-MCDT); subjects with MCI-MCDT were also grouped based on the presence of a memory impairment. Subjects with MCI were classified as possible when there were comorbidities that could explain the subjects' cognitive deficits and as probable when there were none. Main Outcome Measure: Dementia. Results: The incidence of all dementias in the subjects with MCI was 147 per 1000 person-years (mean follow-up overall, 4.3 years). Of the 136 subjects with MCI, 69 (51%) in 1998 through 1999 progressed to dementia (57 [83%] to Alzheimer disease [AD]), but 25 (18%) returned to normal. Of the 10 subjects with probable MCI amnestic type, 7 (70%) progressed to dementia (all of them to AD) and none returned to normal, whereas 7 (41%) of the 17 subjects with possible MCI amnestic type became demented (6 [86%] to AD) and 3 (18%) returned to normal. Of the 40 subjects with probable MCI-MCDT, 21 (52%) progressed to dementia (17 [81%] to AD) and 2 (5%) returned to normal. Of the 69 subjects with possible MCI-MCDT, 34 (49%) progressed to dementia (28 [82%] to AD) and 20 (29%) returned to normal. Among the subjects with probable MCI-MCDT, 15 (54%) of 28 with and 6 (50%) of 12 without memory deficits progressed to dementia. Conclusions: Subjects with MCI are at high risk for dementia. The probable MCI diagnosis identified individuals in the earliest stages of dementia, usually AD, whereas the possible MCI diagnosis identified a more heterogeneous group. However, this latter group had only a slightly lower rate of conversion to dementia than the group with probable MCI, suggesting that even with comorbid conditions, there is a high likelihood of the presence of a progressive dementing disorder.