Changes in flexibility and structural stability of Pseudomonas aeruginosa azurin in response to cavity-creating mutations were probed by the phosphorescence emission of Trp-48, which was deeply buried in the compact hydrophobic core of the macromolecule, and by measurements of guanidinum hydrochloride unfolding, respectively. Replacement of the bulky side chains Phe-110, Phe-29, and Tyr-108 with the smaller Ala introduced cavities at different distances from the hydrophobic core. The phosphorescence lifetime (tau(0)) of Trp-48, buried inside the protein core, and the acrylamide quenching rate constant (k(q)) were used to monitor local and global flexibility changes induced by the introduction of the cavity. The results of this work demonstrate the following: 1), the effect on core flexibility of the insertion of cavities is not correlated readily to the distance of the cavity from the core; 2), the protein global flexibility results are related to the cavity distance from the packed core of the macromolecule; and 3), the increase in protein flexibility does not correspond necessarily to a comparable destabilizing effect of some mutations.