While asthmatics are known to have a greater response to bronchodilators than patients of chronic obstructive pulmonary disease (COPD), whether the pattern of response also differs has not been explored. Forced vital capacity (FVC) and forced expiratory volume in 1st second (FEV1) were measured before and 20 minutes after inhalation of 200 microg salbutamol in patients of bronchial asthma (n=133) and (COPD) (n=116). Three types of responses (defined as > or = 12% and 200 ml increase in FEV1 or FVC) were identified: increase in (i) only FVC (FVC response), (ii) only FEV1 (FEV1 response), and, (iii) both FVC and FEV1 (double response). The mean +/- SEM absolute increase in FEV1 was significantly greater in asthmatics (307+/-17ml) as compared to 120+/-12 ml in COPD patients (p<0.0001). On the other hand, the increase in FVC was not different in the two groups (296+/-22 ml and 230+/-24 ml, respectively, p>0.05). The proportion of subjects showing a > or = 200 ml increase in FEV1 was greater among asthmatics as compared to COPD (p<0.0001) but the proportions showing a > or = 200 ml in FVC were similar (p>0.05). All the three types of responses were observed in asthmatics with a double response being the commonest. In COPD, an FVC response was the predominant response while the FEV1 response was rare. Multinomial logistic regression revealed that younger subjects (below 45 years) were more likely to have a double or exclusive FEV1 response. Greater severity of obstruction was associated with higher odds for each of the three responses, the odds being especially very high for an exclusive FEV1 response. The odds for a double response and an exclusive FEV1 response were significantly increased in asthmatics as compared to COPD. For FVC response, age category and disease were not significant determinants. It was concluded that bronchodilator responsiveness in asthma and COPD differs not only quantitatively but also in the pattern.