Adult polyglucosan body disease (APBD) is a rare disorder, presenting with varying combinations and severity of upper and lower motor neuron dysfunction, sensory deficits, dementia, and urinary incontinence. Onset is in the 40s or 50s. The diagnosis is made by finding polyglucosan bodies (PB) in histologic sections of brain or spinal cord, peripheral nerve, or dermal sweat glands. Although 2 pairs of the 22 previously reported cases were siblings, the familial nature of the disease has not been emphasized.We report 2 adult siblings, a male and a female, each of whom had the clinical diagnosis of vascular dementia. Both were confirmed at autopsy to have APBD. We characterized the lesions in autopsy tissues using a battery of histological stains, lectin histochemistry, and electron microscopy.Innumerable PB were distributed throughout brain, heart, skeletal muscle, liver, and dermal sweat glands. PB were highlighted by periodic acid Schiff stain and concanavalin A lectin. Ultrastructurally, PB were composed of aggregates of filaments within axons and astrocytic processes, and lying free in the neuropil, but not within neuronal perikarya.It is important to consider APBD in cases of familial dementia of unknown etiology. Ante-mortem biopsy of axillary skin may be diagnostic.