The model peptides A(8)K and A(10)K (self-assemble in water into ca. 100 nm long ribbon-like aggregates. These structures can be described as beta-sheets laminated into a ribbon structure with a constant elliptical cross-section of 4 by 8 nm, where the longer axis corresponds to a finite number, N approximate to 15, of laminated sheets, and 4 nm corresponds to a stretched peptide length. The ribbon cross-section is strikingly constant and independent of the peptide concentration. High-contrast transmission electron microscopy shows that the ribbons are twisted with a pitch lambda approximate to 15 nm. The self-assembly is analyzed within a simple model taking into account the interfacial free energy of the hydrophobic beta-sheets and a free energy penalty arising from an increased stretching of hydrogen bonds within the laminated beta-sheets, arising from the twist of the ribbons. The model predicts an optimal value N, in agreement with the experimental observations.