This randomized, placebo-controlled, double-blind study was designed to investigate the short-term efficacy of ultrasound therapy in 90 patients with knee osteoarthritis (OA). Patients were randomly assigned to three groups: group 1 received continuous ultrasound, group 2 received pulsed ultrasound, and group 3 received a 'sham' (placebo) ultrasound for 5 min each session. All treatments were applied once a day for 5 days a week for 2 weeks, i.e. a total treatment duration of 10 days. Compared with baseline, significant improvements were observed in the visual analogue scale pain scores and the Western Ontario and McMaster Universities osteoarthritis index (WOMAC) scores in all three groups. The reductions in pain and WOMAC scores were significantly higher in patients treated with pulsed ultrasound than in the placebo group. In conclusion, pulsed ultrasound therapy is a safe and effective treatment modality in patients with knee OA. Further research is required to investigate the long-term efficacy of pulsed ultrasound therapy in knee OA.