The purpose of this study was to investigate possible rapid effects of light therapy on depressed mood in patients with seasonal affective disorder. Participants received 1 hour of bright light therapy and 1 hour of placebo dim red light in a randomized order crossover design. Depressed mood was measured at baseline and after each hour of light treatment using two self-report depression scales (Profile of Mood States-Depression-Dejection [POMS-D] subscale and the Beck Depression Inventory II [BDI-II]). When light effects were grouped for the two sessions, there was significantly greater reduction in self-report depression scores by -1.3 (p = 0.02) on the BDI-II and -1.2 (p = 0.02) on the POMS-D. A significant but modest improvement was detected after a single active light session. This is the first study, to our knowledge, to document an immediate improvement with light treatment using a placebo-controlled design with a clinical sample of depressed individuals.