Full-dimensional quantum calculations of rovibrational energy levels of the HOOH molecule have been carried out using the Lanczos algorithm and a potential energy surface based on high quality ab initio calculations. The calculated results for Jless than or equal to4 show excellent agreement with available experimental data. The molecule behaves like a rigid asymmetric top for low-lying vibrational states with n(4)less than or equal to3. However, rotational energy levels of higher torsional states deviate significantly from the rigid-rotor model, indicating strong coupling between the torsional and rotational degrees of freedom. Such rotational level patterns should be observable and will provide valuable insight into intramolecular hindered rotation. (C) 2003 American Institute of Physics.