Abstract Aluminum alloy 2024 initially in the T4 (naturally aged) condition and subjected to additional aging at an elevated temperature may exhibit inflected stress-strain hysteresis loops when it is subsequently fatigue cycled at room temperature. We have characterized the development of inflected loops in this material as a function of aging time at 190°C. Inflections are absent in the T4 condition, appear during the first fatigue cycle in underaged material and are strongly developed after aging to peak strength or somewhat beyond. In all cases, the inflections disappear rapidly with continued cycling. Coincident with the appearance of inflected loops there is a dramatic increase in the Bauschinger back stress σ b and the Bauschinger strain β as a function of aging time, and the fatigue response shifts rapidly from pronounced cyclic hardening to moderate cyclic softening. The surface appearance of the material over the final strain cycle changes from sharply defined relatively coarse planar slip at zero stress in the T4 condition to diffuse slip with very little surface offset in material exhibiting a strongly inflected hysteresis loop. These observations are consistent with the development of non-shearable precipitate particles during aging at 190 °C. Glide dislocations stored as Orowan shear loops around such particles during forward deformation may assist reverse deformation at a low stress during the second part of the cycle, giving rise to the inflected hysteresis loops. The hysteresis behavior is shown to be quantitatively in agreement with stress-strain curves derived on the basis of a model of kinematic (memory-dependent) hardening.