OBJECTIVES: : To examine whether the move from the SF-36 to the SF-6D entails a loss in discriminative and evaluative strengths, the magnitude of that loss and whether it matters. METHODS: : The study used relative validity (RV); a ratio of two F statistics, and standardized response means (SRM) to evaluate sensitivity and responsiveness of the SF-36 scales and SF-6D index. An RV of 1 reflected the most sensitive/responsive scale and the smaller the RV the less sensitive the measure would be. Cohen’s criterion for interpreting effect sizes was used to interpret the SRMs. The data used were initially collected for prior studies in seven diseases/conditions: chronic obstructive pulmonary disease, leg ulcers, the elderly in exercise, osteoarthritis, irritable bowel syndrome, migraine and obesity. Identified discriminative and evaluative variables were used to compare RVs and SRMs of the SF-36 scales and the SF-6D index. The mean RV differences and mean SRMs differences between the SF-36 scales and the SF-6D index represented the loss or gain in sensitivity. RESULTS: Data were available from a total of 10,089 subjects. No single SF-36 scale consistently had the largest RV or SRM, and there was no largest RV or SRM observed for the SF-6D index in any condition studied. Comparisons showed the SF-6D index was more discriminative with a mean RV difference of 0.09, (95% CI; 0.07 to 0.12) and more responsive with a mean SRM difference of 0.08, (95% CI; 0 to 0.16) than the SF-36 scales. However, based on longitudinal RVs the index was less responsive with a mean RV difference of 0.07, (95% CI; 0.01 to 0.15) than the SF-36 scales. CONCLUSIONS: : Moving from the SF-36 to the SF-6D index entails a loss in evaluative strength and a gain in discriminative strength, a loss/gain too small to matter given the merits of either instrument.