Light therapy is an effective treatment of seasonal affective disorder (SAD), when administered daily for at least several weeks. We have previously reported a small improvement in mood in SAD patients following exposure to the first hour of treatment. We now reevaluate retrospectively mood changes during shorter exposures comparing depression ratings at baseline, 20, 40, and 60 minutes of light. Participants were 15 depressed patients with SAD, untreated, who were tested during the winter season. The treatment consisted of 10,000 lux of white cool fluorescent light. Depression was measured using the 24-item NIMH scale (24-NIMH). The data were analyzed using ANOVA on ranks and Wilcoxon signed rank tests. Light resulted in significant improvement in mood at every interval when compared with baseline (p< .001). The 40 minute exposure resulted in a greater improvement than the 20 minute exposure (p < .001) but was not different from the 60 minute exposure (p < = .068). We conclude that immediate improvement in mood can be detected after the first session of light with exposures as short as 20 minutes, and that 40 minutes of exposure is not less effective than 60 minutes.