The hydrogen-bond ability of nitrogen in X-CNÁÁÁHO-R systems was investigated using crystallographic data retrieved from the Cambridge Structural Database and via ab initio calculations. The classification of hydrogen-bond geometries by the nature of the donor shows that the strength of the interaction increases in the order water % alcohols `phenols. If the hydrogen-bond geometries are classified according to the nature of the acceptor inside each sample of donor, the strongest hydrogen bonds are always observed for 'push-pull' nitriles, as observed in solution on the pK HB scale for such molecules. Ab initio molecular orbital calculations are then used to compute descriptors of hydrogen-bond basicity (enthalpy DH°, hydrogen-bond length and minimum electrostatic potential of the base) for hydrogen-bond formation between water and 18 nitriles embracing a wide range of structure and basicity. Correlations between the hydrogen-bond basicity scale pK HB and these quantum mechanical descriptors allow the calculation of pK HB values for experimentally inaccessible bases and the treatment of polyfunctional nitriles. They also confirm the super-basicity of the nitrile group of cimetidine, a well known antiulcer drug. This property might be relevant in the molecular recognition of cimetidine by the H 2 receptor. Copyright