The redetermined structure of 1,3,5-trinitro-1,3,5-triaza-cyclo-hexane, C(3)H(6)N(6)O(6), at 90 (2) K has ortho-rhom-bic (Pbca) symmetry. It is of inter-est with respect to energetic compounds. The structure was originally investigated through X-ray diffraction by Hultgren [(1936). J. Chem. Phys.4, 84]. Later X-ray investigations were completed by McCrone [(1950). Anal. Chem.22, 954-955] and Harris, Reed & Gluyas [(1959). AFOSR-TR-59-165 Ohio State University Research Foundation, Columbus, Ohio, USA]. A single-crystal neutron diffraction study was performed by Choi & Prince [(1972). Acta Cryst. B28, 2857-2862] to ascertain the H-atom positions, which had not been defined by the earlier X-ray diffraction studies. All previous studies were performed at or near room temperature. The structure provided is the α polymorph of the title compound. The ring atoms are arranged in the chair conformation with two nitro groups occupying pseudo-equatorial positions and the remaining nitro group is axial. The crystal packing is stabilized by close intramolecular interactions from one H atom in each methylene group to O atoms of adjacent nitro groups, ranging from 2.251 to 2.593 Å.