Oxidatively damaged lipid membranes are known to promote the aggregation of amyloid beta proteins and fibril formation. Oxidative damage typically produces 4-hydroxy-2-nonenal when lipid membranes contain omega-6 polyunsaturated fatty acyl chains, and this compound is known to modify the three His residues in Abeta proteins by Michael addition. In this report, the ability of 4-hydroxy-2-nonenal to reproduce the previously observed amyloidogenic effects of oxidative lipid damage on amyloid beta proteins is demonstrated and the mechanism by which it exerts these effects is examined. Results indicate that 4-hydroxy-2-nonenal modifies the three His residues in amyloid beta proteins, which increases their membrane affinity and causes them to adopt a conformation on membranes that is similar to their conformation in a mature amyloid fibril. As a consequence, fibril formation is accelerated at relatively low protein concentrations, and the ability to seed the formation of fibrils by unmodified amyloid beta proteins is enhanced. These in vitro findings linking oxidative stress to amyloid fibril formation may be significant to the in vivo mechanism by which oxidative stress is linked to the formation of amyloid plaques in Alzheimer's disease.