OBJECTIVE The purpose of this study was to assess the effects of acute joint effusion on balance in patients with knee osteoarthritis. METHODS Forty-four female subjects with painful knee osteoarthritis participated in this single-blind, randomized, controlled clinical trial. All subjects were randomly assigned to either the injection or the control group. A volume of 20 ml of normal saline was injected into the knee joint cavity of the test subjects, who performed static and dynamic balance tests twice before and after joint infusion. The subjects in the control group performed the same tests without joint infusion. The outcome variables for static measurement were the mean speeds (millimeter per second) of the movement of the center of pressure in the mediolateral and anteroposterior directions and the mean velocity moment (square millimeter per second) with both eyes opened and eyes closed conditions. For dynamic measurement, time (second) and distance (millimeter) of the center of pressure were used. RESULTS Significant interaction of the balance-test variables (mean speeds, velocity moment, time, and distance) between the groups and time (P > 0.05) was not found. There were no significant differences between the groups in any of the balance-test variables (P > 0.05). CONCLUSIONS This study showed that acute joint effusion has no effect on static or dynamic postural sway in patients with knee osteoarthritis.