THE TITLE COMPOUND [SYSTEMATIC NAME: 10-methyl-benzo[g]pteridine-2,4(3H,10H)-dione 5-oxide], C(11)H(8)N(4)O(3), consists of a large rigid isoalloxazine group which is approximately planar (r.m.s. deviation = 0.037 Å). In the crystal, inter-molecular N-H⋯O hydrogen bonds link the mol-ecules into centrosymmetric dimers. Dimers related by translation along the c axis form stacks through π-π inter-actions [centroid-centroid distances = 3.560 (5) and 3.542 (5) Å]. Weak inter-molecular C-H⋯O inter-actions further consolidate the crystal packing.