Background: Cognitive deficit by hypoxia and/or hypercapnia is one of neuropsychological impairments frequently observed in patients with chronic obstructive pulmonary disease (COPD). The degree of cognitive deficit is variable among patients with similar level of hypoxia and/or hypercapnia, although a cause of this individual difference is well not known. COPD can be divided into two characteristic clinical entities including predominant emphysema and predominant bronchitis. This study was designed to evaluate the individual difference in cognitive deficit respond to hypoxia and/or hypercapnia in patients with COPD. Method: Sixteen patients with COPD (9 emphysema-dominant and 7 bronchitis-dominant) participated in this study. On admission arterial blood gas analysis and trail-making B (TMB) test for the evaluation of cognitive function were done in all patients. Mean TMB scores and the correlations between TMB scores and arterial blood gases were compared between two clinical groups. Results: 1) Mean TMB scores and arterial blood gases between two clinical groups were not different. 2) There was a tendency to be higher TMB score in hypoxemia, acidemia, and hypercapnia. However these findings were not statistically significant. 3) In emphysema-dominant group, was mostly well correlated with TMB score (r=0.693). 4) In bronchitis-dominant group, arterial pH was mostly well correlated with TMB score (r=-0.526). Conclusion: Our data suggest that the individual difference in cognitive deficit respond to hypoxia and/or hypercapnia in patients with COPD may be dependent on their clinical entities, and arterial blood gases mostly well correlated with cognitive function that may be different according to their clinical entities.