Nine patients with posterior cortical atrophy (PCA), a rare degenerative brain disease of unclear etiology and nosology, were followed over a mean time of 7.4 years. The mean age at onset was low (56.2 years). At onset, eight patients had visuo-spatial and eight had memory impairment. A minority showed early signs of occipital lobe involvement with visual agnosia or hemianopia. Eight patients developed dementia after a mean course of five years. 18F-FDG-PET data of six patients were analysed with statistical parametric mapping. They showed hypometabolism centred on the lateral and medial parietal associative cortex, with variable involvement of the adjacent temporal and occipital associative cortex. A minority showed involvement of the frontal lobes, possibly related to deafferenting of areas related to the control of eye movements. Atrophy and hypometabolism were markedly asymmetric in a subset of cases. Autopsy was performed in one patient. Presenile onset, location, and asymmetry of atrophy suggest that PCA represents a biologically separable variant of Alzheimer's disease.