Many assume that, relative to generic measures, condition-specific health measures are both more sensitive to the condition's severity and more specific because they are less affected by other conditions. We analyzed the sensitivity and specificity of the generic SF-36, condition-specific scales based on the SF-36, and condition-specific measures based on the Knee Society's Clinical Rating System in a study of osteoarthritis patients following knee replacement. As hypothesized, knee-specific role function and pain measures were more specific than generic measures among patients with other comorbid conditions, and less so among patients with only knee problems. Physical function scales of both types were equally specific. Clinical indicators based on x-ray and range of motion were only weakly related to all measures of function.