BACKGROUND AND PURPOSE Aquatic physical therapy is frequently used in the management of patients with hip and knee osteoarthritis (OA), yet there is little research establishing its efficacy for this population. The purpose of this study was to evaluate the effects of aquatic physical therapy on hip or knee OA. SUBJECTS A total of 71 volunteers with symptomatic hip OA or knee OA participated in this study. METHODS The study was designed as a randomized controlled trial in which participants randomly received 6 weeks of aquatic physical therapy or no aquatic physical therapy. Outcome measures included pain, physical function, physical activity levels, quality of life, and muscle strength. RESULTS The intervention resulted in less pain and joint stiffness and greater physical function, quality of life, and hip muscle strength. Totals of 72% and 75% of participants reported improvements in pain and function, respectively, compared with only 17% (each) of control participants. Benefits were maintained 6 weeks after the completion of physical therapy, with 84% of participants continuing independently. DISCUSSION AND CONCLUSION Compared with no intervention, a 6-week program of aquatic physical therapy resulted in significantly less pain and improved physical function, strength, and quality of life. It is unclear whether the benefits were attributable to intervention effects or a placebo response.