BACKGROUND Conversion of hip arthrodesis to a THA reportedly provides a reasonable solution, improving function, reducing back and knee pain, and slowing degeneration of neighboring joints associated with a hip fusion. Patients generally are satisfied with conversion despite the fact that range of mobility, muscle strength, leg-length discrepancy (LLD), persistence of limp, and need for assistive walking aids generally are worse than those for conventional primary THA. OBJECTIVE We compared THA after hip arthrodesis and primary THA to determine whether these procedures would be associated with similar functional scores, maintenance of scores with time, complications and failures, survivorship of the arthroplasty, and patient satisfaction. METHODS We retrospectively matched 48 patients undergoing conversion of a fused hip to a THA between January 1980 and January 2000, with 50 patients receiving a primary THA during the same period. We prospectively followed all patients between January 2000 and January 2010. The changes in function and pain after THA were compared between the two cohorts using the Harris hip score (HHS) and the Rosser Index Matrix (RIM). The Oxford hip score (OHS) and the SF-36 also were used to assess quality of life (QOL) during followup. Complications were collected and survivorship of the THA was evaluated. Patient satisfaction was assessed using the Robertsson and Dunbar questionnaire. The minimum followup was 10 years (mean, 17 years; range, 10-29 years). RESULTS At last followup, hip function and health-related QOL were similar for patients having conversion of hip arthrodesis to THA and for patients having a routine THA. Scores diminished overall in the two groups between 2000 and 2010, but without a difference for the HHS, RIM QOL, and OHS in the study cohort. The rate of complications, THA survival, and patient satisfaction were similar in both groups. CONCLUSIONS Conversion of hip arthrodesis to a THA provides substantial improvement of hip function and health-related QOL, with an acceptable rate of complications, good expectancy of survival for the arthroplasty, and high level of patient satisfaction comparable to those of primary THA. METHODS Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.