The sodium salt of the title compound (1-{[(R)-m-(E)-2-(7-chloroquinol-2-yl)vinyl)]-a-[2-(1-hydroxy-1-methylethyl)phenylethyl]-benzylthio}-methyl)cyclopropane acetic acid (1), known as Montelukast (a free acid), is a potent and selective leukotriene D-4 receptor antagonist. With an annual turnover of over 3 billion US dollars, it still attracts considerable interest in current pharmaceutical research, however, up until now no structural information has been available. The unit cell contains two symmetry independent molecules, which are connected to long ribbons along the c-axis viahydrogen bonds. Lattice energy minimizations reveal that the main part of the lattice energy may be attributed to multipole–multipole interactions between highly polarized Montelukast moieties. However, it seems that the comparable small contributions of the above mentioned hydrogen bonds are essential structure directing ingredients for crystallization.