Objective: To investigate the effects of light therapy on serotonin transporter binding (5-HTT BPND), an index of 5-HTT levels, in the anterior cingulate and prefrontal cortices (ACC and PFC) during winter in seasonal affective disorder (SAD). 5-HTT BPND fluctuates seasonally to a greater extent in SAD relative to health. We hypothesized that in SAD, 5-HTT BPND would be reduced in the ACC and PFC following light therapy. Methods: Eleven SAD participants underwent [C-11] DASB positron emission tomography (PET) scans to measure 5-HTT BPND before and after 2 weeks of daily morning light therapy. Results: The primary finding was a main effect of treatment on 5-HTT BPND in the ACC and PFC (repeated-measures MANOVA, F-(2,F-9) = 6.82, P = 0.016). This effect was significant in the ACC (F-(1,F-10) = 15.11 and P = 0.003, magnitude of decrease, 11.94%) and PFC (F-(1,F-10) = 8.33, P = 0.016, magnitude of decrease, 9.13%). 5-HTT BPND also decreased in other regions assayed following light therapy (repeated-measures MANOVA, F-(4,F-7) = 8.54, P = 0.028) including the hippocampus, ventral striatum, dorsal putamen, thalamus and midbrain (F(1,10) = 8.02-36.94, P < 0.0001-0.018; magnitude -8.83% to -16.74%). Conclusions: These results demonstrate that light therapy reaches an important therapeutic target in the treatment of SAD and provide a basis for improvement of this treatment via application of [C-11] DASB PET.