High cycle fatigue properties of high-pressure die casting magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic fatigue tests up to 109 cycles show mean fatigue limits of approx. 38¨C50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude, Kcr is exceeded. Kcr of the magnesium alloys range from 0.85¡À0.05 to 1.05¡À0.05 MPa¡Ìm, and 1.85¡À0.10 MPa¡Ìm was found for AlSi9Cu3. Below Kcr, fatigue cracks may initiate at porosity, however, do not propagate until failure. Using Kcr, the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.