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The title compound, C19H19N3O3, prepared by condensing 4-aminoantipyrine and 4-methoxy-2-hydroxybenzaldehyde in methanol, is the second monoclinic polymorph of this compound which crystallizes in the space group C2/c. The structure was previously reported [Wang, Zhang, Yan, Zheng & Yang (2007). Acta Cryst. E63, o1245–o1246] in the space group P21/c. The hydroxyl group is disordered over two positions with occupancies of 0.787 (4) and 0.213 (4). The triply substituted benzene ring and the phenyl ring form dihedral angles of 12.2 (2) and 53.7 (2)°, respectively, with the pyrazolone ring; the corresponding values in the P21/c polymorph are 7.5 (2) and 42.6 (2)°. Intramolecular O—H...N and C—H...O hydrogen bonds are observed in the major disorder component. Adjacent molecules are linked through intermolecular O—H...O hydrogen bonds, forming dimers.