The authors examined behavioral and pathophysiologic substrates in a patient with adult-onset Hallervorden-Spatz syndrome who presented with insidious cognitive decline but no motor impairment. The authors combined longitudinal case history and serial neuropsychologic testing with functional neuroimaging (positron emission tomography), structural neuroimaging (magnetic resonance imaging), and brain tissue analyses. Serial assessments of a 29-year-old woman showed progressive dementia. Marked cognitive and behavioral deficits were seen on neuropsychologic testing, corresponding to striking cortical abnormalities on positron emission tomography, magnetic resonance imaging, and histopathologic studies. Typical motor manifestations of the disorder did not emerge until the patient was 34 years old, 5 years after the onset of cognitive symptoms. Hallervorden-Spatz syndrome should be considered in the differential diagnosis of progressive cortical dementia in a young adult, even in the absence of motor dysfunction.