Changes in the mechanical properties of amorphous polycarbonate during the fatigue failure initiation stage lead to overall embrittlement of the polymer. The results indicate that the fracture toughness of the material can be reduced by similar to 35% without visible damage. The ’static’ mechanical characteristics, such as yield stress and strain and absorbed energy, are influenced very modestly by the cyclic fatigue. The observations are explained by the presence of a small number of nanometre-sized voids or ’protocrazes’, detected by transmission and scanning electron microscopy measurements. It is shown that physical ageing and fatigue loading affect the structural state of the polymer differently despite similar effects on the macroscopic mechanical properties.