Several markers of lipid peroxidation are available with different degrees of specificity, from malondialdehyde as a global marker, to F(2)-isoprostane, which is specifically produced from arachidonic acid. Among these, 4-hydroxynonenal is recognized as a breakdown product of fatty acid hydroperoxides, such as 15-hydroperoxy-eicosatetraenoic acid and 13-hydroperoxy-octade cadienoic acid from the n -6 fatty acids. Furthermore, 4-hydroxyhexenal (4-HHE) derives from n -3 fatty acid hydroperoxides. We have recently described the occurrence of 4-hydroxydodecadienal (4-HDDE) from the 12-lipoxygenase product of arachidonic acid 12-hydroperoxy-eicosatetraenoic acid. These three hydroxy-alkenals may be measured in human plasma by GC-MS, but they may partly be generated in the course of sampling, and the relative volatility of 4-HHE makes its measurement quite unreliable. We have successfully characterized and measured the stable oxidized carboxylic acid products from the hydroxy-alkenals 4-HNA, 4-HHA and 4-HDDA in urine. The ratio between 4-HHA and 4-HNA found in the same urinary sample might provide useful information on the location of lipid peroxidation, accounting for the high enrichment of the cerebrovascular system with docosahexaenoic acid, the main n -3 fatty acid in humans.