Three-dimensional (3D) printing is used to manufacture custom acetabular cups to treat patients with massive acetabular defects. There is a risk of defects occurring in these, often in the form of structural voids. Our aim was to investigate the presence of voids in commercially available cups. We examined 12, final-production titanium custom acetabular cups, that had been 3D-printed by six manufacturers. We measured their mass, then performed micro-computed tomography (micro-CT) imaging to determine their volume and density. The micro-CT data were examined for the presence of voids. In cups that had voids, we computed (1) the number of voids, (2) their volume and the cup volume fraction, (3) their sphericity, (4) size, and (5) their location. The cups had median mass, volume, and density of 208.5 g, 46,471 mm³, and 4.42 g/cm³, respectively. Five cups were found to contain a median (range) of 90 (58–101) structural voids. The median void volume and cup volume fractions of cups with voids were 5.17 (1.05–17.33) mm³ and 99.983 (99.972–99.998)%, respectively. The median void sphericity and size were 0.47 (0.19–0.65) and 0.64 (0.27–8.82) mm, respectively. Voids were predominantly located adjacent to screw holes, within flanges, and at the transition between design features;​ these were between 0.17 and 4.66 mm from the cup surfaces. This is the first study to examine defects within final-production 3D-printed custom cups, providing data for regulators, surgeons, and manufacturers about the variability in final print quality. The size, shape, and location of these voids are such that there may be an increased risk of crack initiation from them.