Crystalline polymorphs of 1-ethyl-3,7-dihydro-8-[(1R,2R) -(hydroxycyclopentyl)amino]-3-(2-hydroxyethyl)-7-[(3-bromo-4-methoxypheny- l)methyl]-1H-Purine-2,6-dione in Form 1 and Form 2, which exhibit x-ray powder diffraction profiles substantially the same as those shown in FIGS. 5 and 6, respectively, and which exhibit differential scanning calorimtery profiles substantially the same as those shown in FIGS. 2 and 4, respectively, and are represented by the formula: ##STR00001## Pharmaceutical compositions comprising the polymorph Form 1 or 2 of Compound 13 and at least one excipient or carrier, and methods of using the polymorph Form 1 or 2 of Compound 13 to treat a variety of physiological disorders, such as erectile dysfunction.