Maintained hypoxia has been reported to induce inactivation of the Angiotensin Converting Enzyme (ACE). Variations have also been observed in patients with chronic obstructive pulmonary disease (COPD) who have chronic hypoxemia and loss of the vascular endothelium. OBJECTIVE 1) to determine serum ACE activity in patients with COPD treated with and without continuous ambulatory oxygen therapy (CAOT); 2) to verify whether there is a correlation between ACE and any hematological, spirometric or gasometric parameter. METHODS fifty-eight patients fulfilling clinical and spirometric parameters of COPD were studied. Patients were assigned to two groups of therapy: A) Group A, without Continuous Ambulatory Oxygen Therapy (CAOT): 31 males and 1 female (mean age: 64.8 +/- 6.52). B) Group B (with CAOT): 23 males and 3 females (mean age years: 63.76 +/- 8 years). The following procedures were performed: spirometry, gasometry, blood chemistry, and serum ACE measurements by means of a radioenzymatic assay. The Student "t" test with the Bonferroni correction and Pearson regression analysis were used for the statistical analysis. RESULTS significant differences were observed for ACE values between Group A and Group B: 42.81 +/- 11.30 vs. 33.40 +/- 9.43 mumol/min/l, with a p value of 0.001, and also between Group B and reference values: 33.40 +/- 9.43 vs. 39.70 +/- 9.65 mumol/min/l, with a p value of 0.002. No differences were observed between Group A and reference values. No correlations wer found between ACE and any of the variables studied. CONCLUSIONS ACE was not decreased in all patients with COPD. ACE was decreased only in patients with COPD and respiratory insufficiency requiring CAOT and with advanced disease. This results can be correlated with changes in vascular endothelium, pulmonary parenchyma and metabolism. It could be a marker of poor prognosis or advanced disease.