An indexed powder diffraction pattern and related crystallographic data are reported for secnidazole [C7H11N3O3, IUPAC name: 1-(2-hydroxypropyl)-2-methyl-5-nitroimidazole], which is not represented in the Powder Diffraction File. The unit cell dimensions were determined from diffractometer methods, using monochromatic CuKα1 radiation, and evaluated by indexing programs. The monoclinic cell found for 1-(2-hydroxypropyl)-2-methyl-5-nitroimidazole is: a=12.426(2) Å, b=12.173(2) Å, c=6.656(1) Å, β=100.19(1)°, Z=4, space group P21/c (No. 14), Dx=1.271 g/cm3. Crystallization of an anhydrous powdered sample of secnidazole in a buffer solution of Na2B4O7 and NaOH (pH 10.4) resulted in crystals that contained water of crystallization, as shown by single crystal structure determination. Secnidazole exhibits crystal pseudopolymorphism, because the experimental powder pattern of the anhydrous form and the calculated pattern from the structure determination of the hydrate form are similar. Observed powder diffraction data for this drug were interpreted with the aid of a calculated pattern based upon the crystal structure determined. The cell found by TREOR90P for anhydrous secnidazole is in good agreement with that of the hemihydrate form determined from single crystal diffraction: a=12.424(2) Å, b=12.187(2) Å, c=6.662(1) Å, β=100.9(1)°; Z=4.