An analysis of backbone hydrogen bonds has been performed on nine high-resolution protein X-ray crystal structures. Backbone hydrogen-bond geometry is compared in the context of X-ray crystal structure resolution. A strong correlation between the hydrogen-bond distance, R-HO, and the hydrogen-bond angle, theta(NHO), is observed when the X-ray crystal structure resolution is <1.00 Angstrom. Ab initio calculations were performed to substantiate these results. The angle and distance limits found in our correlation for the backbone hydrogen-bond geometry can be used to evaluate the quality of protein structures and for further NMR structure refinement.