Purpose : Clinical trials require scales which are sensitive to the effects of intervention. This study examined the sensitivity to change of three generic health status measurement scales commonly used in evaluations of interventions in chronic, disabling disease. Methods : Questionnaires comprising the SF-36, London Handicap Scale and Nottingham Extended Activities of Daily Living scale were administered to 81 patients before and after hip replacement. Changes were quantified as 'effect sizes'. Results : Eighty nine per cent of patients reported improvements three months after surgery. The largest changes were seen on the SF-36 pain scale (effect size 1.2 at three months, 1.5 at 6-12 months), physical function (1.1, 1.3) and role limitation - physical (0.8, 1.2) scales. The London Handicap Scale also measured large changes (effect sizes 0.6, 0.9), but the Extended Activities of Daily Living scale was insensitive to change (effect sizes 0.1, 0.2). Conclusions : Some of the SF-36 dimensions were very sensitive to change. The London Handicap Scale was also reasonably responsive, and has the advantage of being a single, utility-based, score. The simpler Extended ADL score was poorly responsive.