We describe a 61-year-old woman who gradually developed deficits of balance, gait, and the ability to negotiate movement in space, together with an unusual pattern of cognitive deficits. A series of non-invasive investigations over three years including EEG, CT, MRI, PET and serial neuropsychological review had not provided a diagnosis. Significantly, the four neuropsychological assessments had revealed no progressive decline in cognition. Brain biopsy revealed an abundance of corpora amylacea, and a diagnosis of adult polyglucosan body disease (APBD) was made. This case contributes to the body of knowledge about the cognitive manifestations of this rare disease, and the stability of its functional impact over time.