Images obtained by high-resolution X-ray tomography are used to show experimentally the three-dimensional character of short fatigue cracks in a cast aluminium alloy. The pores present in this alloy are preferred initiation sites for fatigue cracks, but no correlation can be established with the three-dimensional size of the pores. A strong interaction between fatigue cracks and grain boundaries is clearly shown, both at the surface and in the bulk, using post mortem 3D observations as well as in situ observations.