Described herein are amorphous and crystalline forms of the androgen receptor modulator 4-[7-(6-cyano-5-trifluoromethylpyridin-3-yl)-8-oxo-6-thioxo-5,7-diazaspiro[3.4]oct-5-yl]-2-fluoro-N-methylbenzamide. Also described are pharmaceutical compositions suitable for administration to a mammal that include the androgen receptor modulator, and methods of using the androgen receptor modulator, alone and in combination with other compounds, for treating diseases or conditions that are associated with androgen receptor activity.