Background: It is unknown whether there is an optimal time for surgery - that is, whether waiting until a patient has greater disability results in a worse outcome. We examined the effect of baseline status on the outcome of total hip arthroplasty to determine if such a relationship existed. Methods: All Medicare patients undergoing total hip arthroplasty for osteoarthritis in twelve states were identified by the fiscal intermediary beginning in October 1994 and continuing for approximately eight months; 1640 eligible patients were identified. Consenting patients were surveyed within two months after the procedure and again at twelve months after the procedure, and their medical records were reviewed. The baseline survey, completed by 1120 patients, included items regarding the level of activity, the presence and severity of pain with walking, the need for assistance for walking, the distance that the patient could walk, and whether the patient could perform Instrumental Activities of Daily Living (IADLs). Data on comorbid diagnoses were collected from the medical records. Results: Patients with pain during walking at baseline were more likely to have pain at one year than those without pain at baseline (21% compared with 9%; p < 0.05). Patients who needed assistance with walking at baseline were more likely to need assistance at one year than those who did not need assistance at baseline (38% compared with 15%; p < 0.01). Similar results were seen with regard to the need for assistance with housework (39% compared with 18%; p < 0.01) and grocery shopping (37% compared with 14%; p < 0.01) and, to a lesser extent, for overall participation in moderate activity (17% compared with 10%; p < 0.01). These results persisted after controlling for comorbidities in a multiple regression equation. However, the worse a patient's preoperative status, the more he or she gained in all four measures (p < 0.01). Conclusions: The findings of the present study suggest that the worse a patient's preoperative status, the more he or she may benefit from total hip arthroplasty. However, our findings also suggest that patients who have a worse preoperative status may not have as good an outcome as those who have a better preoperative status. Patients and physicians should consider these findings when discussing the timing of total hip arthroplasty.