The fused five- and six-membered rings in the title compound, C(10)H(10)N(2)O, are approximately coplanar, with an r.m.s. deviation of 0.008 Å. The mean plane of the allyl group is roughly perpendicular to the mean plane of the 1,3-benzimidazol-2(3H)-one system, making a dihedral angle of 86.1 (2)°. In the crystal, each mol-ecule is linked to its symmetry equivalent partner by a pair of N-H⋯O and C-H⋯O hydrogen bonds.