The objective of this study was to examine the association among the duration of COPD, degree of hypoxemia, and neurological abnormalities including cognitive functioning. Fifty-four patients with severe COPD and 24 age- and sex-matched controls, were included in the study. All patients and controls were administered pulmonary function tests, standardized Mini-mental State Examination (MMSE), Blessed Dementia Scale (BDS), Physical Self-maintenance Scale (PSMS), Modified Activities of Daily Living scale (MADL), Instrumental Activities of Daily Living scale (IADL), Cornell Scale for Depression in Dementia (CSDD), Global Deterioration Scale (GDS) and Clinical Dementia Rating (CDR). In addition, detailed physical and neurological examinations were performed. Sixty-four percent of patients with COPD showed abnormalities in MMSE, predominantly in recent memory, construction, attention, language, and orientation domains. Functional abnormalities were correlated with cognitive abnormalities. Although COPD patients did not show significant depression compared to controls, 77.7%. of the patients showed subjective and objective cognitive disturbance and 72.2% of the patients were classified as questionable or mild dementia. In conclusion, patients with COPD show significant cognitive and functional impairments that cannot be explained just by coincidence or by depression.