Based upon the thermodynamic formulation for linearly viscoelastic fracture developed earlier, an extension has been made to fatigue fracture resulting from repeated (oscillatory) load applications. The theoretical analysis of internal spherical‐flaw growth, due to a uniformly distributed radial oscillatory input of displacement, predicts a growth‐rest cycle whose characteristics depend upon the mechanical properties of the medium. The results for this idealized problem are compared to experimental data for crack growth in a precracked sheet specimen subjected to oscillatory displacement input, and a qualitative similarity is observed. It is therefore believed that the analytical model is representative, and its study can reveal the main features of macroscopic flaw growth.