The title compound, C(18)H(15)N(3)O, is the product of the thermal decomposition of the diazo-nium salt derived from 2-amino-N-methyl-N-(3-methyl-1-phenyl-1H-pyrazol-5-yl)benzamide. It is characterized by a trans orientation of the methyl groups with respect to the tricyclic ring system. The mol-ecule has a nearly planar phenyl-pyrazolo[3,4-c]isoquinolin-5-one system, the largest deviation from the mean plane being 0.066 (2) Å for the O atom. The dihedral angle between the phenyl substituent and the heterotricycle is 67 (1)°. The packing is stabilized by C-H⋯N hydrogen-bond inter-actions, with the formation of mol-ecular chains along the c axis.