Millimetre wave therapy (MWT) is a promising complementary method for pain relief, however rigorous investigations of its effectiveness are needed. The purpose of this study was to examine if MWT can reduce opioid requirement compared to sham procedure applied for relief of acute pain in patients after total knee arthroplasty (TKA). Eighty patients undergoing TKA were randomly assigned to receive MWT or sham procedure. Patients and evaluators were blinded to the group allocation. MWT consisted of six sessions, each session of 30 min duration. During each session the knee wound was exposed to electromagnetic waves with frequency 50-75 GHz and power density 4.2 mW/cm(2). Postoperative analgesia with piritramide, a weak opioid with 0.7 potency of morphine delivered via patient-controlled analgesia pump, was directed to achieve pain intensity of less than 40 on a 100 mm visual analogue scale (VAS). The primary outcome measure was postoperative piritramide requirement for three days after surgery. Secondary outcome measures were: total ibuprofen requirement from the fourth postoperative day to discharge; success of patients' blinding; patients' satisfaction with pain relief; incidence of analgesia-related side effects; heart rate and blood pressure. Piritramide requirement was similar in both groups whereby all patients reported adequate pain relief measured on a VAS. Secondary outcome measures were also comparable in both groups. The majority of patients in both groups believed they had received true MWT and wanted to repeat it in future. Millimetre waves applied to surfaces of surgical wounds did not reduce opioid requirement compared to the sham procedure after TKA.