This study represents a long-term effort to find optimal techniques for evaluating outcome in patients who have undergone total joint arthroplasty. Sensitivity of five health status questionnaires was studied in a longitudinal evaluation of orthopedic surgery. The questionnaires (Arthritis Impact Measurement Scales [AIMS], Functional Status Index [FSI], Health Assessment Questionnaire [HAQ], Index of Well Being [IWB], and Sickness Impact Profile [SIP]) were administered to 38 patients with end-stage arthritis at three points in time: two weeks before hip or knee arthroplasty, and at three-month and 12- to 15-month follow-up. Response values (i.e., changes within patients) were calculated on four scales: global health, pain, mobility, and social function. By the three-month follow-up, most instruments detected large mean responses in global health, pain scores, and mobility. Smaller changes on these scales were found between three and 12 to 15 months. Social function showed small to modest gains at successive follow-ups. Standardized response means were calculated to assess sensitivity to detect change. Confidence intervals for these indices were constructed using a jackknife procedure, and significance tests were performed by pairing selected indices. Finally, the study projected sample sizes required to assess a new therapy, using each response. These statistical tools facilitated comparisons among instruments and may prove useful in other settings.