Ultrasound (US) therapy is used to reduce pain and inflammation and to accelerate healing after soft tissue injury. However, there is little objective evidence of its effectiveness and the mechanisms which may cause these effects are unknown. In a placebo-controlled double-blind clinical trial we examined the contribution of placebo and massage effects in ultrasound therapy following bilateral surgical extraction of lower third molars. Four to 6 h after surgery the patients (25 per group) received either no therapy, US (0.1 W/cm2), 'mock' US with massage, 'mock' US without massage, or 'self-massage' with a dummy applicator. Facial swelling, trismus, serum C-reactive protein, serum cortisol, pain and anxiety were measured 24 h postoperatively. The results showed that the beneficial analgesic and anti-inflammatory effects of US therapy were placebo-mediated, with maximum effect in the placebo ('mock' US) group without circular massaging with the applicator). Self-massage by the patient produced no significant effect. This placebo action was independent of changes in serum cortisol or patient anxiety state. US therapy can significantly reduce postoperative morbidity, but by placebo-mediated mechanisms which are unrelated to the US itself.