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A relative decrease intake of n-3 to n-6 fatty acids has been implicated in the pathogenesis of depression. However, the associations between different sources of dietary n-3, n-6, and their ratio, and the risk of depression have not been prospectively studied. We prospectively studied 54,632 women who were 50 to 77 years of age and free from depressive symptoms at baseline. Information on diet was obtained from validated food frequency questionnaires (FFQ) completed four times before baseline (1984, 1986, 1990, 1994). Clinical depression was defined as reporting both physiciandiagnosed depression and regular antidepressant medication use. Relative risks (RR) of clinical depression, adjusted for age and other possible risk factors, were estimate using Cox-proportional hazard models. During 10 years of follow-up (1996-2006), 2,823 incident cases of clinical depression were documented. Long-chain n-3 intake from fish was not associated with depression risk (multivariate RR for 0.3 g/day increment=0.99 [95% CI: 0.88, 1.10]). Intake of alpha-linolenic acid (ALA) was not associated with depression risk, except in multivariate model adjusted for n-6 linoleic acid (LA). For each 0.5 g/day increment of ALA, multivariate RR was 0.82 (95% CI, 0.71 to 0.94). Interaction between ALA and LA was found to be significant for depression risk (P=0.02). Within quintiles of LA, a 0.5 g/day increment in ALA was inversely associated with depression in the first, second and third LA quintiles (RR=0.57 (95% CI, 0.37 to 0.87); 0.62 (95% CI, 0.41 to 0.93); 0.68 (95% CI, 0.47 to 0.96) respectively) but not in the fourth and fifth quintiles. The results of this large longitudinal study do not support a protective effect of long-chain n-3 from fish or fish intake on depression risk. However, this study provides support for the hypothesis that higher ALA and lower LA intakes might reduce depression risk, but this relation warrants further investigation.