Here, the authors present the cryo-EM structure of in vitro amyloid fibrils from recombinant SAA1.1 protein that were formed by seeding with fibrils purified from systemic AA amyloidosis tissue. This in vitro fibril structure resembles the structure of the ex vivo fibrils but differs from unseeded in vitro fibrils. These findings show that fibril morphologies can be propagated in vitro by seeding.