Cognitive function, psychological status, and attitudes were investigated in 90 patients with chronic obstructive pulmonary disease before the initiation of long-term oxygen therapy and after one year of treatment. Assessment included clinical interview, Wechsler Intelligence Scale I.Q., Bourdon-Wiersma Test, Benton Verbal Retention Test, Rey's Test of Remembering 15 Words, Beck's Depression Scale, Taylor's Manifest Anxiety Scale, and Tylka's Psychological Evaluation Scale of the Effectiveness of Rehabilitation. The mean I.Q. of the patients studied was slightly above average at 107 points. Tests of cognitive function showed reduced performance. These included tests of repeating numbers, pictorial anecdotes and numerical symbols. Rey's Test of Word Memory was also below average but increased following treatment. In the Bourdon Test, there were 58 correct deletions and 7 omissions initially, improving to 67 and 8, respectively, after treatment. Poor visual and spatial memory did not improve after treatment. Before treatment, patients demonstrated depressed mood, low self-esteem with narrow interests, signs of anxiety, mental stress and depression. After a year of long-term oxygen therapy significant improvements in mood and attitudes were demonstrated. We conclude that long-term oxygen therapy may be capable of producing a significant improvement in emotional status. However, the effects of oxygen treatment are difficult to separate from effects of other aspects of care in producing a sense of increased security and well-being.