Background: Bright visible-spectrum light therapy has proven effective in the treatment of seasonal affective disorder (SAD) and recent basic research suggests that blue wavelengths ∼470 nm account for that effectiveness. To more stringently test the importance of these wavelengths, bright red-light was used for the placebo (control) condition. Methods: Thirty subjects meeting DSM-IV criteria for SAD were randomized to narrow-band light-emitting diode panels emitting blue- or red-light in this 3-week, parallel, double-blind trial. Twenty-five subjects participated in an open-label blue-light follow-up. Subjects were divided in a blinded, post hoc manner into two groups: SAD only and those experiencing depression with seasonal intensification. The outcome was assessed using Hamilton Depression Rating Scale–17 item version (HAMD-17) and the Structured Interview Guide for the Hamilton Depression Rating Scale—SAD version. Responders were defined by Clinical Global Impression—Improvement scale. Results: HAMD-17 scores improved more under the blue-light condition (51%) than under the red-light condition (32%) (P=.05). Further, in the blue arm 60% of subjects responded compared with 13% in the red arm (P=.01). During the open-label phase, subjects from both double-blind arms improved over baseline. SAD alone patients responded numerically better to treatment than those experiencing depression with seasonal intensification during both treatment periods. Conclusions: Narrow bandwidth blue-light therapy proved superior to red-light therapy. Blue-light therapy produced results similar to both previous 10,000 lux visible-spectrum light studies and many medication studies. The use of bright red panels supported claims that wavelengths of ∼470 nm account for the documented effectiveness of light therapy. Depression and Anxiety, 2009. © 2008 Wiley-Liss, Inc.