(±)11-HDHA is an autoxidation product of docosahexaenoic acid (DHA) in vitro. It is also produced from incubations of DHA in rat liver, brain, and intestinal microsomes. DHA is metabolized to 11(S)-HDHA by human platelets and canine retina. In addition to 11(S)-HDHA, 14(S)-HDHA is also produced by platelets. 11(S)-HDHA was shown to be an inhibitor of U-46619-induced human platelet aggregation and rabbit and rat aortic smooth muscle contraction with IC50 values of about 50, 4.7, and 7.5 μM, respectively. (±)11-HDHA is a potential marker of oxidative stress in brain and retina where DHA is an abundant polyunsaturated fatty acid.