During earthquakes, steel reinforcing bars in reinforced concrete structures may be subjected to large inelastic deformation in tension and compression as high as 6% strain, eventually leading to low-cycle fatigue failure (Mander, Panthaki, & Kasalanati, 1994). This was observed in laboratory testing and post-earthquake damage inspections including during the Mw 7.8 Kaikoura earthquake in New Zealand (NZ) (see Figure 1 and Figure 2) (El-Bahy, Kunnath, Stone, & Taylor, 1999b; El-Bahy, Kunnath, Stone, & Taylor, 1999a; Palermo et al., 2017). Earthquakes are usually preceded and/or followed by other events of larger or smaller intensity; longitudinal steel failures may not occur during a first event, but in a subsequent one due to the cumulative damage. Seismic events can also occur several months apart and during this period, if the steel has experienced any post-yielding deformation during the first event, strain ageing takes place, modifying the mechanical properties of the material. In this document, fatigue lives for unaged and aged 12-mm diameter NZ-manufactured Grade 300E reinforcing bars are compared.