Tibial shaft fractures have a high incidence of delayed and non-union, often requiring multiple procedures. Pulsed electromagnetic field (PEMF) stimulation is a safe and effective treatment for tibial non union but little is known about the efficacy of PEMF in preventing non-union in acute fractures. Between August 2005 and December 2008, eligible patients presenting at six major metropolitan hospitals with acute tibial shaft fractures (AO classification 4-2) were included. Participants were randomised by an independent allocation centre to identical active or inactive PEMF devices, both prescribed to be worn ten hours daily for twelve weeks. Baseline data were collected, and patients and surgeons were surveyed serially via post and/or phone at three, six and twelve months. Participants, data collectors and data analysts were blinded. The main outcome was any surgical intervention for delayed/non union. Data from 150 patients (76 active and 74 inactive) were available for analysis (12 month follow up 78%). There was no between-group difference in age, gender, mechanism of injury, open grade, smoking status, or compliance with the device (hours of use). Overall compliance was moderate (5.7 hours daily use). Fifteen of 76 (20%) of active device patients required surgery for delayed/non-union vs. 8 of 74 (11%) in the placebo group; odds ratio of surgery=2.03, p=0.13, 95% CI 0.80 to 5.12. An intention to treat analysis adjusted for open grade and hours the device was used showed an odds ratio of requiring surgery of 1.78 (p=0.24, 95% CI 0.68 to 4.64). A sub-group analysis of 80 compliant patients (defined a priori as a minimum average of six hours daily use) was performed. Nine of 35 (26%) patients with an active device required surgery for delayed/non-union vs. 3 of 45 (7%) in the placebo group; odds ratio of surgery=7.60 (p=0.017, 95% CI 1.43 to 40.24). Based on these unexpected initial results, PEMF stimulation does not prevent revision surgery for delayed/non-union in acute fractures of the tibial shaft