The molecular structure, packing properties, and intermolecular interactions of two structural polymorphs of N-palmitoylethanolamine (NPEA) have been determined by single-crystal X-ray diffraction. Polymorphs α and β crystallized in monoclinic space group P21/c and orthorhombic space group P bca, respectively. In both polymorphs, NPEA molecules are organized in a tail-to-tail manner, resembling a bilayer membrane. Although the molecular packing in polymorph α is similar to that in N-myristoylethanolamine and N-stearoylethanolamine, polymorph β is a new form. The acyl chains in both polymorphs are tilted by ∼35° with respect to the bilayer normal, with their hydrocarbon moieties packed in an orthorhombic subcell. In both structures, the hydroxy group of NPEA forms two hydrogen bonds with the hydroxy groups of molecules in the opposite leaflet, resulting in extended, zig-zag type H-bonded networks along the b-axis in polymorph α and along the a-axis in polymorph β. Additionally, the amide N-H and carbonyl groups of adjacent molecules are involved in N-H⋯O hydrogen bonds that connect adjacent molecules along the b-axis and a-axis, respectively, in α and β. Whereas in polymorph α the L-shaped NPEA molecules in opposite layers are arranged to yield a Z-like organization, in polymorph β one of the two NPEA molecules is rotated 180°, leading to a W-like arrangement. Lattice energy calculations indicate that polymorph α is more stable than polymorph β by ∼2.65 kcal/mol. Copyright © 2006 by the American Society for Biochemistry and Molecular Biology, Inc.