A randomized double blind clinical trial was conducted to determine the effectiveness of ultrasound (US) therapy in knee osteoarthritis (OA). Sixty-seven patients (mean age 54.8 +/-7) were randomized to receive either 1 MHz frequency or 1 watt/cm(2) power continuous ultrasound for 5 min (n = 34) or sham US (n = 33) as a placebo. Ten sessions of treatment were applied to the target knee of the patient. A blinded evaluation at baseline and after treatment was made. Primary outcome was pain on movement assessed by visual analog scale (VAS). Secondary outcomes consisted of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores and 50 meters walking time. Both groups showed significant improvements in knee pain on movement. In the treatment group, the improvement in VAS score was statistically and significantly higher (p < 0.001) and more pronounced than in the placebo group. Pain reduction averaged 47.76% in the treatment group (p = 0.013). Secondary outcomes improved in both groups but reached statistical significance only in the treatment group: p = 0.006 for the mean change in total WOMAC scores and p = 0.041 for 50 meters walking time. Results suggest that therapeutic US is safe and effective treatment modality in pain relief and improvement of functions in patients with knee OA.