The crystal structures of efavirenz, systematic name: (S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one, C14H9ClF3NO2, (I), a potent and specific inhibitor of HIV-1 reverse transcriptase, and its monohydrate, C14H9ClF3NO2 center dot H2O, (II), have been determined by single crystal X-ray diffraction (XRD) analysis. The molecular conformation in both structures has a crane bird like appearance. This structure analysis illustrates the influence of the self-complementary amide group in crystal packing through its involvement in forming helical hydrogen-bonding catemers in both structures. The water molecule in (II) has a cohesive function connecting the efavirenz molecules and, more significantly, forms one-dimensional water chains along the crystallographic a axis, thus providing a possible insight into the role of lattice water in drugs and its formulations.