Background: Bright light therapy (BLT) influences the regulation of melatonin and is an established treatment for seasonal affective disorder (SAD). This study was designed to examine the efficacy of BLT for seizure control in adults with focal epilepsy. Design: 101 adults with medically intractable focal epilepsy were recruited to a parallel-design, double-blind, randomized trial of BLT as an add on treatment for epilepsy. Methods: All patients monitored their seizure frequency at home for a 12-week baseline period (September 2010 to December 2010). 51 of the participants were allocated with a high-intensity (HI) light box emitting 10,000 lx using an automated permuted block randomization grid. 50 were allocated with a cosmetically identical box emitting 2000 lx (low-intensity - LI), a subtherapeutic dose in other patient populations. Both groups were instructed to use their box for 20-30 min, upon waking every day for 12 weeks (January-March 2011). The primary outcome measure for the trial was seizure frequency. Results: 77 participants (39 high-intensity/38 low-intensity) completed the trial and returned adequate data for analyses. Median reduction of seizures during the treatment phase was 1.5 in the LI group and 3 in the HI group (p >0.05). 6 patients (15%) in the high-intensity condition experienced an overall reduction of 50% or more in the frequency of complex partial seizures compared to 9 (24%) patients who used the low intensity light box. Response rates in the HI and LI treatment conditions were not significantly different (p >0.05). Patients with hippocampal sclerosis were more likely to respond to BLT at either intensity than patients with other focal epilepsies (risk ratio = 1.7 95% CI lower limit = 0.6, upper limit = 4.6). Conclusions: We did not find a significant difference in the responder rates in the low- vs. high-intensity arms of the trial. Some patterns within the data suggest that BLT may warrant further investigation as a treatment for people with hippocampal pathology. Our initial findings suggest that caution should be exercised in using BLT in people with extra temporal focal epilepsy as it may result in an increase in seizures for some. (c) 2012 Elsevier Inc. All rights reserved.