Background:The N-terminal fragment of amyloidogenic apoA-I mutants deposits as fibrils by unknown mechanisms. Results: The G26R mutation partially prevents helix formation of the N-terminal fragment upon lipid binding, thereby facilitating ␤-transition and fibril formation. Conclusion: Membrane binding modulates fibril formation of apoA-I through partially destabilized helical conformation. Significance: The results reveal a new pathway for amyloid fibril formation by apoA-I.