Objective: Adult polyglucosan body disease (APBD) is an autosomal recessive glycogen storage disease resulting in intracellular accumulation of polyglucosan bodies in the nervous system and cerebral demyelination. Approximately 50 APBD patients have been described in the literature and nearly 50% exhibited “cognitive decline.” However, few studies employed standardized neuropsychological tests, and those that did were not comprehensively described. Method: This case characterizes the neuropsychological profile of a 46-year-old Caucasian male with APBD. The patient was initially diagnosed with and treated for multiple sclerosis; APBD was confirmed biochemically and genetically 5-years later after clinicians learned of a family history of glycogen storage disease. Presenting complaints included increasing urinary urgency/incontinence, gait/balance problems, fatigue, and concentration difficulties. MRI revealed symmetric white matter changes in cerebellar peduncles, periventricular regions, the posterior limb of internal capsule, and medial lemnisci of medulla and pons. Results: Reduced initiation/fluency, complex attention/working memory, and sustained attention were prominent features of the neuropsychological profile. As cognitive complexity increased, mild but consistent reductions in flexibility, planning and problem-solving emerged. Cognitive strengths included visuospatial analysis, reasoning, language, and learning/memory. Illness-related maladjustment was also observed. Conclusion: This case sheds needed light on neuropsychological characteristics of APBD, a rare and potentially underdiagnosed autosomal recessive leukodystrophy. The observed pattern of executive dysfunction provides further evidence that cerebellar white matter degeneration and lesions can produce cognitive profiles indicative of frontal systems involvement. Finally, although this cognitive profile is consistent with other more common demyelinating conditions, clinicians are encouraged to take note of how critical family history was in reaching accurate diagnosis.