OBJECTIVE To quantify patients' functional improvement in the immediate postoperative period after total hip arthroplasty (THA) or total knee arthroplasty (TKA). METHODS A prospective observational study design was used to examine the number of physical therapy treatment sessions and postoperative days required to attain independence in 4 critical functional milestones, with independence defined as the ability to perform the activity without the assistance of another person. The 4 functional milestones selected were the ability to perform supine to sit transfers; sit to stand transfers; ambulation to 100'; and the ability to climb stairs. Consecutive patients undergoing elective THA or TKA for the first time at a community teaching hospital were entered into the study. RESULTS The study group consisted of 81 patients undergoing elective THA or TKA (33 THA and 48 TKA). There was wide variability in the number of physical therapy sessions and postoperative days required to attain each of the 4 milestones and all 4 milestones. Climbing stairs was the most difficult task, followed by walking 100', performing sit to stand transfers, and performing supine to sit transfers. The achievement of the latter 2 milestones was similar. CONCLUSION Our examination of patients' progress in attaining specific functional milestones is a unique approach to outcome assessment that emphasizes the physical therapy aspect of postsurgical rehabilitation. A wide variability in patients' functional progress during the acute care admission after elective THA or TKA was demonstrated. The reasons for this variability need to be explored since they may have important implications for planning rehabilitation related to THA, TKA or other orthopedic reconstructive procedures.