Spirometric tests of 30 patients with chronic obstructive pulmonary disease (COPD) or asthma were evaluated just before and 15 minutes after the administration of 200 mg of salbutamol by inhalation. All patients had a significant increase os their airflow after the bronchodilator administration. In 15 patients the increase of FEV1 was greater than 13% (Group I) and in other 15 patients the increase of FEF25-75% was greater than 23% (Group II). The two groups showed differences with respect to the initial FEV1, and FEF25-75% (% of predicted), with values significantly lower (p 0.01) in Group I. There was no significant difference between the two Groups concerning the age, sex and the initial FVC (% for predicted).