Health status (or Health-Related Quality of Life) measurement is an established method for assessing the overall efficacy of treatments for asthma and chronic obstructive pulmonary disease (COPD). Such measurements can indicate the potential clinical significance of a treatment's effect. This paper is concerned with methods of estimating the threshold of clinical significance for three widely used health status questionnaires for asthma and COPD: the Asthma Quality of Life Questionnaire, Chronic Respiratory Questionnaire and St George's Respiratory Questionnaire. It discusses the methodology used to obtain such estimates and shows that the estimates appear to be fairly reliable; ie. for a given questionnaire, similar estimates may be obtained in different studies. These empirically derived thresholds are all mean estimates with confidence intervals around them. The presence of these confidence intervals affects the way in which the thresholds may be used to draw inferences concerning the clinical relevance of clinical trial results. A new system of judging the magnitude of clinically significant results is proposed. Finally, an attempt is made to translate these thresholds into scenarios that illustrate what a clinically significant change with treatment may mean to an individual patient.