A manual crystallization screen was performed on 3-oxauracil and 5-hydroxyuracil (isobarbituric acid), culminating in the first determination of their crystal structures. Concurrently but independently, the low energy crystal structures of these molecules were computed by a search for minima in the lattice energy. The crystal structure of 3-oxauracil corresponded to the global minimum in the lattice energy, with an unusually large energy gap of 4 kJ mol⁻¹ between the observed and other hypothetical crystal structures. Therefore, this structure was easily predicted despite some inadequacies in the computational model. The combination of the experimental and computational search suggests that this is the most thermodynamically stable anhydrous crystal structure of 3-oxauracil and it seems unlikely that it will have any readily produced polymorphs. The experimental crystal structure of 5-hydroxyuracil was also found as a low energy crystal structure in the search but a few other hypothetical structures with different hydrogen bonding motifs were predicted to be thermodynamically competitive. It is therefore possible that other polymorphs might be found for 5-hydroxyuracil, although they were not found in this crystallization screen. These successful crystal structure predictions illustrate that the confidence with which crystal structures and polymorphism can be predicted varies between structurally similar molecules.