The complete three-dimensional structure of the alpha-amylase inhibitor Tendamistat in aqueous solution was determined by 1H nuclear magnetic resonance and distance geometry calculations using the program DISMAN. Compared to an earlier, preliminary determination of the polypeptide backbone conformation, stereo-specific assignments were obtained for 41 of the 89 prochiral groups in the protein, and a much more extensive set of experimental constraints was collected, including 842 distance constraints from nuclear Overhauser effects and over 100 supplementary constraints from spin-spin coupling constants and the identification of intramolecular hydrogen bonds. The complete protein molecule, including the amino acid side-chains is characterized by a group of nine structures corresponding to the results of the nine DISMAN calculations with minimal residual error functions. The average of the pairwise minimal root-mean-square distances among these nine structures is 0.85 A for the polypeptide backbone, and 1.52 A for all the heavy atoms. The procedures used for the structure determination are described and a detailed analysis is presented of correlations between the experimental input data and the precision of the structure determination.