A growing multibillion dollar industry markets magnetic necklaces, bracelets, bands, insoles, back braces, mattresses, etc., for pain relief, although there is little evidence for their efficacy. We sought to evaluate the effect of magnetic therapy on pain intensity and opioid requirements in patients with postoperative pain. We designed a randomized, double-blind, controlled trial. One-hundred-sixty-five patients older than 12 yr of age were randomized to magnetic (n = 81) or sham therapy (n = 84) upon reporting moderate-to-severe pain in the postanesthesia care unit. Devices were placed over the surgical incision and left in place for 2 h. Patients rated their pain intensity on a 0-10 scale every 10 min and received incremental doses of morphine until pain intensity was < or =4 of 10. Pain intensity levels were similar in both groups. The magnet group had on average 0.04 U more pain intensity (95% confidence interval, -0.4 to 0.5) than the sham group. Opioid requirements also were similar in both groups. The active magnet group required 1.5 mg more morphine (95% confidence interval, -1.8 to 4.0) than the sham magnet group. Magnetic therapy lacks efficacy in controlling acute postoperative pain intensity levels or opioid requirements and should not be recommended for pain relief in this setting.