High pressure die-casting (HPDC) is widely used as a cost-effective way to massproduce metal components that are required to have close dimensional tolerances and smooth surface finishes. Approximately 50%, by mass, of the aluminium castings produced worldwide are made by this manufacturing route. However, HPDC components are relatively porous compared with other types of castings and so cannot usually be conventionally heat treated to improve mechanical properties. This follows because during solution treatment (e.g. at 540°C for 8h), the pores expand, resulting in unacceptable surface blisters, distortion and poor mechanical properties. Recent work within the CSIRO Light Metals Flagship has revealed a heat treatment procedure by which the problems of blistering and distortion can be avoided . As a result, large improvements in strength have been achieved, as compared with the as-cast condition. One uncertainty is the behaviour of heat treated HPDCs under cyclic stress and this paper investigates the fatigue properties of a common high pressure die-casting alloy, A380 (Al-8.5Si-3.5Cu). Comparisons are made between as-cast, T4 and T6 conditions. Fatigue strength is highest for the alloy aged to a T6 temper and ratios of fatigue strength to tensile strength for the as-cast, T4 and T6 conditions are constant at a value of approximately 0.6, which is particularly high for aluminium alloys.