OBJECTIVE: To determine the effectiveness of pulsed electromagnetic fields (PEMF) in the treatment of knee osteoarthritis (OA). MEHTODS: In this randomized, placebo-controlled, double-blind trial, 51 patients with knee OA, diagnosed according to the criteria of the American College of Rheumatology, were treated with PEMF or placebo. Eleven patients failed to attend after screening and were excluded from analysis. Treatment consisted of 3 half-hour periods of exposure per week over 6 weeks in a specially designed cylindrical device that emits low-frequency pulsed electromagnetic fields (25 gauss, 12 Hz). The primary outcome measure was reduction in pain on movement using a 10 cm visual analog scale. Secondary outcome measures included joint swelling and tenderness, the Lequesne index, and overall evaluations of improvement by the patient and examining physician. Evaluations were made at baseline, 3 week and 6 week during treatment and 4 weeks after finishing treatment. RESULTS: There were no significant differences between PEMF and placebo groups in respect of any outcome measures after treatment. Range of motion and knee swelling tended to be improved in the PEMF group. There were no clinically relevant adverse effects attributable to PEMF treatment. CONCLUSION: These results suggest that the PEMF treatment has no clinically significant benefits in patients with knee OA resistant to conventional treatment. The larger studies are needed to confirm the efficacy of PEMF therapy in knee OA.