The invention comprises (1) anhydrous crystalline forms of N-[1-(2-ethoxyethyl)-5-(N-ethyl-N-methylamino)-7-(4-methylpyridin-2-yl-amino)-1H-pyrazolo[4,3-d]pyrimidine-3-carbonyl]methanesulfonamide, (2) pharmaceutical compositions comprising at least one such form, (3) methods for the treatment of a phosphodiesterase-5-mediated condition using at least one such form, and (4) methods for preparing such forms. The compound N-[1-(2-ethoxyethyl)-5-(N-ethyl-N-methylamino)-7-(4-methylpyridin-2-yl-amino)-1H-pyrazolo[4,3-d]pyrimidine-3-carbonyl]methanesulfonamide has the following structure (I).