The three-dimensional structure of the alpha-amylase inhibitor Tendamistat determined by nuclear magnetic resonance in aqueous solution is compared with the Tendamistat crystal structure refined at 2.0 A resolution. Between the two independently obtained structures the root-mean-square distances are 1.05 A for the backbone atoms N, C alpha and C', 1.25 A for the backbone and the interior side-chains, and 1.84 A for all heavy atoms. These numbers show that the interior of the molecule is nearly identical in the two states. Near the protein surface a small number of local differences between the two structures were identified. In most surface areas the solution structure appears more disordered than the crystal structure, with the exception of Tyr15, which was not observed in the X-ray diffraction.