Specimens of pure Cu and dilute Cu-Au (100 ppm Au) were irradiated with fast neutrons (>0.1 MeV) at 4 K and isochronally annealed to 400 K. Recovery spectra were analysed for several induced-defect concentrations varying from ∼1 to 250 ppm in these specimens. Certain existing controversies relating to the recovery of fast- or thermal-neutron-irradiated materials could be satisfactorily explained if the variations in defect concentration were taken into account. The relative concentrations of irradiation-induced defects (CF) and substitutional impurities, viz.CF/CI, were seen to have a major role in the interpretations. Satisfactory comparisons could be made with recent similar studies involving thermal- and fast-neutron irradiations of Pt and its dilute alloys.