Three previously unknown crystal structures (polymorphs) of 2-amino-5-nitropyrimidine (I-III), as determined by single-crystal X-ray diffraction, are reported. Crystal packing, hydrogen-bond patterns, and intermolecular forces have been examined in detail, and theoretical studies were performed on I-III to estimate their lattice energies and to determine the role of changes in molecular conformation upon the resulting crystal structure. Each structure is characterized by an extensive network of hydrogen bonds (including C-H ... N and C-H ... O interactions) and short interlayer distances. This structural system has shown up limitations of several assumptions that are generally made in current methods of predicting crystal structures and polymorphism, but within these limitations, the applied scheme has been successful.