The optimal method of assessing the effect of treatment on the day-to-day function of patients with chronic airflow limitation is not established. Therefore, we examined the performance of the Six Minute Walk test, a rating of dyspnea following the test and three different questionnaires measuring dyspnea in daily activities, in a controlled trial of inhaled salbutamol and oral theophylline in 24 patients with primarily fixed chronic airflow limitation. Clinically important and statistically significant effects of salbutamol and theophylline on dyspnea during day-to-day activities were detected by each measure, but the Chronic Respiratory Questionnaire (CRQ) appeared more powerful than either the Oxygen Cost Diagram or the Medical Research Council Dyspnea Questionnaire as modified by the Rand Corporation. Changes in the CRQ dyspnea score showed a higher correlation with changes in spirometry, walk test score, dyspnea following the walk test, and global ratings of dyspnea than did the other two measures. Results of the walk test demonstrated statistically significant salbutamol and theophylline effects, but suggested that no added benefit could be obtained with both drugs; the rating of dyspnea following the walk test, however, was consistent with an additive effect. We conclude that the CRQ is a responsive, valid measure of functional status for clinical trials in chronic lung disease, and that when the six minute walk is used as a measure of outcome, dyspnea following the walk test should be measured.