Key neuropathological changes associated with late-onset dementia are not fully understood. Population-based longitudinal studies offer an opportunity to step back and examine which pathological indices best link to clinical state. CC75C is a longitudinal study of the population aged 75 and over at baseline in Cambridge, UK. We report on the first 213 participants coming to autopsy with sufficient information for an end of life dementia diagnosis. Clinical diagnosis was ascertained by examining retrospective informant interviews, survey responses, and death certificates according to DSM-IV criteria. The neuropathological protocol was based on the Consortium to Establish a Registry of Alzheimer's Disease (CERAD). Clinical dementia was present in 113 participants (53%): 67% with Alzheimer's disease, 4% vascular dementia, 22% mixed dementia, and 1% dementia with Lewy bodies. As Alzheimer-type pathology was common, the mutually blinded clinical and neuropathological diagnoses were not strongly related. Multivariable analysis identified associations between dementia during life and entorhinal cortex neuritic plaques, hippocampal diffuse plaques, neocortical neurofibrillary tangles, white matter pallor, Lewy bodies, and hippocampal atrophy. These results were consistent in those with clinical Alzheimer's disease. Vascular pathologies, especially microinfarcts, were more common in those with clinical diagnoses including vascular dementia. Alzheimer-type and cerebrovascular pathology are both common in the very old. A greater burden of these pathologies, Lewy bodies, and hippocampal atrophy, are associated with a higher risk of, but do not define, clinical dementia in old age.