At room temperature, natural polypeptides exposed to high concentrations of a strong denaturant nearly attain the circular dichroism spectra characteristic of random coils. As temperature is increased, the spectra begin to show the signature of a substantial fraction of extended chain, the structure common in beta-sheets. This structural propensity at high temperature is not altered by concentration changes over a greater than 1000-fold range, so it is not caused by aggregation. Four proteins with different folds and varying amounts of alpha-helical and beta-sheet secondary structure, in the presence or absence of denaturant, all were subject to extended chain formation upon heating. This effect arises naturally from the steric constraints associated with polypeptides and is probably counteracted, not enhanced, by hydrophobic interactions. Molecular dynamics simulations in the 298-1000 K range reveal an attractive potential of mean force in the extended chain region of the Ramachandran diagram, which broadens as the temperature is raised. We also demonstrate a direct correlation between extended structure content and the rate of aggregation kinetics. Thus pre-existing extended structure could funnel proteins into aggregates.