Supplementation with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) has been reported to reduce lipid peroxidation products formed from arachidonic acid (F2-isoprostanes) in healthy humans, as well as in those under oxidative stress. α-Linolenic acid (ALA) is a precursor to EPA and DHA; however, its conversion in humans is thought to be inefficient. ALA can also undergo free radical oxidation, forming compounds known as F1-phytoprostanes, which are found in all plants and are in high concentrations in plant pollens. In this study, we examined the effect of ALA supplementation on plasma and urine F1-phytoprostane and F2-isoprostane concentrations in men. Thirty-six nonsmoking men, aged 20–65 y, were recruited from the general population and randomly allocated to consume 9 g/d of either flaxseed oil (62% ALA, 5.4 g/d) or olive oil (placebo) for 4 wk in a parallel design. At baseline and after 4 wk of supplementation, blood samples and a 24-h urine sample were collected for measurement of plasma and urinary F1-phytoprostanes and F2-isoprostanes and plasma fatty acids. Compared with the olive oil group, plasma phospholipid ALA was greater (P <0.0001), as were F1-phytoprostanes in plasma (P = 0.049) and urine (P = 0.06) in the flaxseed oil group after 4 wk supplementation. Flaxseed oil did not affect plasma or urinary F2-isoprostanes. The greater plasma F1-phytoprostane concentration in the flaxseed oil group most likely resulted from the increased plasma concentration of the ALA substrate and/or the F1-phytoprostane content of the flaxseed oil. Future studies are needed to determine the physiological importance of increased plasma and urine F1-phytoprostanes and their relevance to heart disease prevention.