The molecular chaperone Hsp90 sequesters oncogenic mutants of the tumor suppressor p53 that have unstable core domains. It is not known whether p53 is bound in an unfolded, partly folded, or distorted structure, as is unknown for the structure of any bound substrate of Hsp90. It is a particularly difficult problem to analyze in detail the structures of large complexes in which one component is (partly) unfolded. We have shown by transverse relaxation-optimized NMR spectroscopy combined with cross-correlated relaxation-enhanced polarization transfer (CRINEPT-TROSY) that p53 core domain bound in an approximately 200-kDa complex with Hsp90 was predominantly unfolded lacking helical or sheet secondary structure. This mode of binding might be a general feature of substrates of Hsp90.