Accurate rovibrational levels for HCN up to 24 349 cm(-1) above the bottom of the vibrational well and vibrational levels at the saddle point of the HCN/HNC isomerization reaction up to 32 809 cm(-1) above the saddle point have been computed and used to obtain partition functions over the temperature range 200-2500 K. Under the rigid-rotor approximation, the rovibrational partition function for HCN is found to be exactly separable into vibrational and rotational contributions to first order. Two approximate approaches, second-order perturbation theory and simple perturbation theory, the latter of which obviates the need for a direct summation over energy levels, are shown to yield vibrational partition functions for HCN and at the saddle point that agree with the accurate values within 2%. In contrast, the usual harmonic approximation leads to errors of up to 24% in the individual partition functions, resulting in differences of between 3.5% and 6.5% in conventional transition state theory rate constants calculated with harmonic versus anharmonic vibrations.