Ths object of this study was to examine validity, meaningful effect sizes, and patterns of response of the Transition Dyspnea Index (TDI) in a clinical trial cohort of chronic obstructive pulmonary disease (COPD) patients. The design was a retrospective analysis of data from a randomized, double-blind placebo-controlled clinical trial. We analyzed fifty clinical investigation sites in United States. There were 921 patients with stable COPD. Tiotropium 18 microg dry powder or matching placebo was used. Patients were allowed to remain on usual care less ipratropium bromide. Construct validity was demonstrated by significant correlations (P <.05) between Baseline Dyspnea Index (BDI) and other baseline measures, as well as between TDI and changes in other measures at the end of 1 year. Concurrent validity was observed by the significant correlation between TDI and dyspnea diary responses. Changes in TDI focal score were in the range of one unit when the group was stratified by a minimal change in the physician's global evaluation. Significantly less (P <.05) supplemental albuterol was observed in the group of responders defined by a one-unit improvement in TDI. Responders also had few exacerbations and better health status. The validity of the TDI is supported in a large clinical trial setting. A one-unit change in the TDI focal score represented the minimal important difference.