Transthyretin (TTR) binds amyloid-β (Aβ) and may reduce brain Aβ, a pathological feature in Alzheimer's disease (AD). N - 3 fatty acids (FA), docosahexaenoic (DHA), and eicosapentaenoic acid (EPA) may increase TTR transcription in rat hippocampus. We studied effects of n - 3 FA supplementation on TTR-levels in patients with AD. Outpatients were randomized to receive 1.7 g DHA and 0.6 g EPA (n - 3/n - 3 group) or placebo (placebo/n - 3 group) during 6 months. After 6 months, all patients received n - 3 FA for another 6 months. TTR and FA were measured in plasma in all subjects, whereas TTR in cerebrospinal fluid (CSF) was measured in a subgroup. The study was completed by 89 patients in the n - 3/n - 3 group (75 y, 57% w) and 85 in the placebo/n - 3 group (75 y, 46% w). Baseline plasma-TTR was within normal range in both groups. After 6 months, plasma-TTR decreased in the placebo/n - 3 group (p < 0.001 within and p < 0.015 between the groups). No changes were observed in CSF TTR. From 6 to 12 months when both groups were supplemented, plasma-TTR increased significantly in both groups. Repeated measures ANOVA indicated an increase in TTR over time (p = 0.04) in those receiving n - 3 FA for 12 months. By linear regression analyses, n - 3 FA treatment was independently associated with increased plasma-TTR at 6 months (β = -0.172, p = 0.028). Thus, n - 3 FA treatment appeared to increase plasma-TTR in patients with AD. Since TTR may influence Aβ deposition in the brain, the results warrant further exploration.