The objective of the study was to compare the validity of asthma-specific and generic health outcome measures in relation to changes in the severity of asthma and to treatment. Adult patients (n = 142) participating in a randomized placebo-controlled trial at six clinics were assessed at baseline, prior to the withdrawal (placebo) or continuation of treatment with Vanceril and again after 8 weeks. The criterion measures of change in severity included pulmonary function expressed as the percent predicted FEV1, five physician-assessed asthma severity measures (cough, chest tightness, wheezing, shortness of breath and overall condition) and two patient-assessed severity measures (night-time symptoms and overall symptoms). The 8 week change scores were estimated for all generic and specific measures and the results were compared across groups of patients who did and did not change in terms of clinical criteria of disease severity and across treatment groups. The responsiveness of each generic and specific measure was estimated independently using the relative validity (RV) methodology, which compares F-ratios for the mean change scores across measures in analyses of the same comparison groups. RV coefficients estimate how much worse each measure discriminated between comparison groups, relative to the best measure (RV = 1.0). Four standardized asthma-specific measures and a total scale score (based on the Marks questionnaire), an individualized asthma-specific scale measuring limitations in activities most important to each patient (based on the Juniper method) and two newly-developed scales measuring physical and psychosocial symptoms were used as outcome measures, generic health outcome measures included eight functional health and well-being scales as well as the physical and mental health summary scales from the SF-36 health survey. A standardized asthma-specific scale was most valid in discriminating between groups of patients who did and did not change according to all of the clinical criterion variables studied and in discriminating between treated and untreated groups. Different scales performed best, depending on the clinical criterion. The asthma-specific Marks breathlessness scale was significant in all nine comparisons (RV = 0.62-1.0) and was most valid in discriminating between groups in six of nine tests. The overall scale also performed well in all comparisons (RV = 0.58-1.0). The newly-developed physical symptoms scale was significant in discriminating between groups in eight out of nine tests (RV = 0.52-1.0) and was most valid in three of the nine, including the treatment comparison. The psychosocial impact scale discriminated significantly in eight of the nine comparisons (RV = 0.16-0.38), but was less valid than other specific measures. The asthma-specific individualized activities scale discriminated significantly in seven of the nine tests, but performed less well than the other specific measures (RV = 0.21-0.35) and was not significant in the treatment comparison. One or more SF-36 scales discriminated significantly between groups in all nine comparisons. Two of those scales (physical functioning and role-physical) were consistently more valid than the others (RV = 0.17 and 0.58, respectively) and were the only two generic scales that discriminated between groups of patients defined in terms of changes in FEV1 (RV = 0.26-0.58). The SF-36 physical summary scale discriminated significantly between groups in all nine comparisons (RV = 0.19-0.61) and was the most valid generic measure in the treatment comparison (RV = 0.55). The SF-36 mental summary scale was significant only for the two patient-assessed changes in disease severity (RV = 0.31 and 0.32) and for physician-assessed overall severity (RV = 0.12). A comprehensive battery of generic and specific measures is likely to be most useful in understanding the impact of changes in disease severity on the functional health and well-being of adults with asthma, a