There are two mol-ecules in the asymmetric unit of the title compound, C(15)H(17)N(3)O(6). The 2,3-dioxopiperazine ring adopts a half-chair conformation with torsion angles of -7.6 (4) and 35.1 (4)° in one mol-ecule, and 5.3 (4) and 45.4 (4)° in the other mol-ecule. In the crystal structure, the carb-oxy groups are involved in classical inversion-related O-H⋯O hydrogen bonds, which link the mol-ecules into centrosymmetric dimers. These dimers are further linked by inter-molecular O-H⋯O and C-H⋯O hydrogen bonds. Each independent mol-ecule also exhibits an intra-molecular N-H⋯O hydrogen bond. The H atoms of the carb-oxy groups are disordered over two positions, with refined site-occupancy factors of 0.5.