The effects of electron correlation on the calculated isotropic hyperfine coupling, a, in the electronic ground state, X (2)A ", Of HNCN are investigated in an ab initio molecular orbital study. Radical structures obtained from large-basis-set second-order Moller-Plesset perturbation theory (UMP2) calculations and extrapolated from smaller-scale coupled-cluster calculations by Tao et al. [J. Chem. Phys. 1994, 100, 3691] are employed. Large changes in the computed isotropic splittings at the magnetic nuclei, H-1, C-13, and N-14, are observed between the zero-order unrestricted Hartree-Fock (UHF) model and the approximate UMP2 correlation treatment. Further electron correlation effects on the calculated splittings must thus be considered and here are determined from the results of coupled-cluster doubles (UCCD) and quadratic configuration interaction (UQCISD) treatments with moderately large basis sets. Still higher correlation effects due to triple and quadruple replacements in the UHF reference determinant are recovered perturbatively. An investigation of the basis set dependence of the computed magnetic coupling constants is carried out at the UHF and UMP2 levels of theory. Taken together, these calculations suggest values of a(H-1) = -51 MHz, a(N-14) = 25 MHz, a(C-13) = -70 MHz, a(N-14(outer)) = 13 MHz. which may be of use in the unraveling of complex rotational spectra assigned to this species.