Background: Seasonal affective disorder (SAD) is characterized by recurrent episodes of major depression with a seasonal pattern, treated with light therapy (LT). Duration of light therapy differs. This study investigates retrospectively whether a single week of LT is as effective as two weeks, whether males and Females respond differently, and whether there is an effect of expectations as assessed before treatment. Methods: 83 women, and 25 men received either one-week (n=42) or two weeks (n=66) of LT were included in three studies. Before LT, patients' expectations on therapy response were assessed. Results: Depression severity was similar in both groups before treatment (F(1,106)=0.19 ns) and decreased significantly during treatment (main effect "time" F(2,105)=176.7, p.- 0.001). The speed of therapy response differs significantly in treatment duration, in favor of 1 week (F(2,105)=3.2, p= 0.046). A significant positive correlation between expectations and therapy response was found in women (rho=0.243, p=0.027) and not in men (rho = -0.154, ns). When expectation was added as a covariate in the repeated-measures analysis it shows a positive effect of the level of expectation on the speed of therapy response (F(2,104)=4.1, p=0.018). Limitations: A limitation is the retrospective design. Conclusions: There is no difference between 1 and 2 weeks of LT in overall therapy outcome, but the speed of therapy response differed between 1 week LT and 2 weeks LT. Together with the significant correlation between expectations and therapy response in women, we hypothesize that expectations play a role in the speed of therapy response. (C) 2014 Elsevier B.V. All rights reserved.