BACKGROUND AND PURPOSE: Understanding the reliability of selected measurement tools is a prerequisite to understanding the effects of clinical interventions. The aim of this investigation was to determine the reliability of the 50-Foot Timed Walk (50 FTW) and 30-second Chair Stand Test (30 CST) in subjects awaiting joint replacement surgery of the hip or knee. METHODS: Eighty-two subjects participating in a 6-week exercise programme were assessed at baseline, 7 weeks and 15 weeks. Four trials of the 50 FTW and two trials of the 30 CST were completed at each assessment. Eleven trained assessors completed the assessments. RESULTS: Intra-class correlations were consistently high for the 50 FTW and 30 CST at all assessments. At the baseline assessment, trial 1 was found to be significantly different from subsequent trials for both the 50 FTW and 30 CST. This effect was not evident at the 7-week and 15-week assessments. At the baseline assessment, scores for the 50 FTW became stable after the first trial. Estimates of minimum detectable change indicated that participants needed to change by more than 3.08 seconds and 1.64 stands to be 90% confident that a real change had occurred for the 50 FTW and 30 CST, respectively. CONCLUSION: The 50 FTW and 30 CST can be reliable measures of physical performance. However, because we found a practice effect at the baseline assessment, a practice trial should be allowed before data collection begins. Because only two trials of the 30 CST were completed, further research is required to confirm whether scores at the initial assessment become stable on repeated testing.