The axial slice width for the GE Advance PET scanner has previously been reported to be worse for 3D acquisitions than for 2D. The goals of this study were to investigate the source(s) of this observed difference and to assess whether the 3D axial slice width could be significantly improved by acquisition and simultaneous reconstruction of axially interleaved data. The axial slice width was measured for the three acquisition modes of the Advance scanner ('standard' high-sensitivity 2D, high-resolution 2D and 3D), with the septa both extended and retracted. A significant degradation in the axial slice width for 3D compared with that for high-sensitivity 2D mode was seen. Near the centre, this difference can largely be attributed to septa collimation effects of the 2D data. At larger radial positions, axial mispositioning of cross-coincidences in 2D acquisitions overshadows the effects of septa collimation, while 3D reconstruction effects also become more important. The axial slice width was estimated to improve by 0.3-1.1 mm with interleaving. This modest improvement would be accompanied by an increase in image noise, since an axial filter with a higher cut-off would be required in the 3D reconstruction to achieve this resolution in the image.