Pulsed electromagnetic field (PEMF) treatment is a non-invasive technique which has wide use in promoting healing of delayed union and non-union of bone. According to reports in the literature, PEMF has a 'success' of about 70%, but with no clear-cut reason to explain the failures. Our tests were carried out on 11 patients with radius non-unions and delayed unions; the results suggest that PEMF failure is associated with implanted metallic plates. In our view, this can be explained because the conducting plates create a uniform bone biopotential around the fracture and thus prevent the negative polarization which stimulates callus formation. Although further controlled and randomized clinical tests are needed, our results indicate that it may be necessary to remove the plates before PEMF application.