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A prospective longitudinal evaluation of 12 patients over a 16-year interval using clinical neurological and imaging data to determine whether posterior cortical atrophy syndrome (PCA) related to early-onset Alzheimer disease (AD) and to examine its natural history. Our 12 patients had a median age of onset of 56 years (range, 48 to 63 y) and were followed for a median of 6 years (range, 3 to 9 y). Patients either presented with complex visual phenomena or developed them with time. Six patients underwent flurodeoxyglucose and Pittsburgh investigational compound B imaging which showed a mismatch between metabolic activity and amyloid deposition with reduced metabolism in parieto-occipital regions on flurodeoxyglucose positron emission tomography and diffuse neocortical uptake of amyloid without occipital predominance. All patients progressively deteriorated using a quality of life and total functional capacity assessments and this change is similar to the natural history of other early-onset AD variants (typical amnestic presentation, logopenic, and frontal). Two patients had neuropathologic assessments and were shown to have AD using standard pathologic criteria. Of interest, 5 of our 12 patients had occupations strongly dependent on visuospatial functioning. PCA is a syndrome that is most likely a variant of early-onset AD and our correlative clinical, structural, functional, and amyloid imaging data, along with neuropathologic studies in 2 patients, support this concept. The natural history of PCA shows progression with time and this trajectory seems to reflect that of other variants of early-onset AD.