We present a classical trajectory study of the photodissociation of HNCO in the S-1 electronic state using a five-dimensional potential energy surface with all atoms confined to a plane. The potential energy surface has been determined by ab initio calculations (multi-reference configuration-interaction method, triple-zeta basis set). Because any coupling to other electronic states is neglected in our calculations, direct comparison with experimental data is limited and basically restricted to energies well above the HN(a (1)Delta)+CO channel. Nevertheless, some aspects are described in a realistic way and help to interpret experimental results. In particular, the calculated vibrational-rotational state distributions of the diatomic fragments are in good agreement with the measured ones. Moreover, our calculations support the most recently published data on the H-atom quantum yield for photolysis with 193-nm photons. It is found that pre-excitation of the HN stretching mode by several quanta significantly enhances the lifetime with respect to breaking the NC bond, which may partly explain the drastic increase of the H-atom quantum yield in the vibrationally mediated photodissociation.