Conventional 1-form crystals of 5-hydroxy-1-methylhydantoin contain,remaining therein in a considerable amount, the organic solvent used in apurification step. In contrast, in II-form crystals, the amount of the organicsolvent remaining therein is smaller than the detection limit. Namely, theII-form crystals contain substantially no residual organic solvent. The novelII-form crystals of 5-hydroxy-1-methylhydantoin obtained throughrecrystallization from water not only contain substantially no residualorganicsolvent, but also have a high bulk density and are hence advantageous forpharmaceutical preparation. They further have properties advantageous forproduction, such as low adhesion. The crystals are significantly useful as amaterial for medicines required to have high safety, such as drugs for renalinsufficiency.
