The title compound, C(6)H(8)N(4)O(3)·2H(2)O, which crystallized as a dihydrate, has two almost planar segments viz. the pyrimidine ring and the C-N-C(=O)-C group [maxmum deviations of 0.020 (2) and 0.014 (2) Å, respectively], with a dihedral angle of 87.45°. In the crystal, the components are linked by O-H⋯O and N-H⋯O hydrogen bonds.