BACKGROUND: Current pharmacologic therapies for patients with osteoarthritis are suboptimal. OBJECTIVE: To determine the efficacy of Curcuma longa extract (CL) for reducing knee symptoms and effusion-synovitis in patients with symptomatic knee osteoarthritis and knee effusion-synovitis. DESIGN: Randomized, double-blind, placebo-controlled trial. (Australian New Zealand Clinical Trials Registry: ACTRN12618000080224). SETTING: Single-center study with patients from southern Tasmania, Australia. PARTICIPANTS: 70 participants with symptomatic knee osteoarthritis and ultrasonography-defined effusion-synovitis. INTERVENTION: 2 capsules of CL (n = 36) or matched placebo (n = 34) per day for 12 weeks. MEASUREMENTS: The 2 primary outcomes were changes in knee pain on a visual analogue scale (VAS) and effusion-synovitis volume on magnetic resonance imaging (MRI). The key secondary outcomes were change in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain and cartilage composition values. Outcomes were assessed over 12 weeks. RESULTS: CL improved VAS pain compared with placebo by -9.1 mm (95% CI, -17.8 to -0.4 mm [P = 0.039]) but did not change effusion-synovitis volume (3.2 mL [CI, -0.3 to 6.8 mL]). CL also improved WOMAC knee pain (-47.2 mm [CI, -81.2 to -13.2 mm]; P = 0.006) but not lateral femoral cartilage T2 relaxation time (-0.4 ms [CI, -1.1 to 0.3 ms]). The incidence of adverse events was similar in the CL (n = 14 [39%]) and placebo (n = 18 [53%]) groups (P = 0.16); 2 events in the CL group and 5 in the placebo group may have been treatment related. LIMITATION: Modest sample size and short duration. CONCLUSION: CL was more effective than placebo for knee pain but did not affect knee effusion-synovitis or cartilage composition. Multicenter trials with larger sample sizes are needed to assess the clinical significance of these findings. PRIMARY FUNDING SOURCE: University of Tasmania and Natural Remedies Private Limited.