The crystal structure of azide-inhibited bovine Cu,Zn superoxide dismutase has been studied and refined based on X-ray synchrotron radiation data, in conjunction with difference Fourier and restrained crystallographic refinement techniques. The final R-factor for the 20,756 reflections in the 10.0 to 2.1 A resolution range is 0.166. In both enzyme subunits, the azide anion, which is a competitive inhibitor expected to mimic the superoxide binding mode, is observed directly coordinated to the Cu2+ at the place of the metal-bound water molecule, forming an ion pair with the conserved active site residue Arg141. The coordination sphere of Cu2+ is partly altered with respect to the uninhibited enzyme: a displacement of 0.67 A in subunit A, and 0.37 A in subunit B of the dimeric enzyme is observed for the Cu2+. Only two ligands in the Cu2+ coordination sphere (His46 and His118) are affected by azide binding, whereas virtually no rearrangement of the Zn2+ ligands is reported.