We present data from two studies which clarify the relationship between the responsiveness and validity of instruments designed to measure health status in clinical trials. In a controlled trial of long vs short duration adjuvant chemotherapy for women with Stage II breast cancer, the Breast Cancer Chemotherapy Questionnaire (BCQ) proved valid as a measure of subjective health status and was able to distinguish long vs short arms. Well validated measures of physical and emotional function developed by the Rand Corporation were unable to distinguish between the two groups. The Eastern Co-operative Oncology Group Criteria (ECOG) distinguished the two groups, but failed criteria of clinical sensibility as a measure of subjective health status. In a study of patients with Crohn's disease and ulcerative colitis, the Inflammatory Bowel Disease Questionnaire (IBDQ) showed small intrasubject variability over time. Gobal ratings of change showed moderate to high correlations with changes in IBDQ score, and patients who reported overall improvement or deterioration showed large changes in IBDQ score. Each of these findings support, in different ways, the reproducibility, validity, and responsiveness of the questionnaire. While the same data can at times bear on both validity and responsiveness, when assessing evaluative instruments it is useful to make a conceptual distinction between the two.