We present quantum mechanical wave packet calculations for the vibrational predissociation of cis-CH3ONO in the S1 state including three degrees of freedom-the CH3O-NO dissociation bond, the N=O stretching coordinate, and the CH3O-N-O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational-rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several quasibound vibrational states of CH3ONO(S1) as well as several final vibrational states of the NO product. In general, the classical trajectory calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely classical treatment in reproducing subtle quantum effects if the dissociation proceeds through a relatively long-lived complex. While the calculations yield satisfactory agreement with the experimental NO state distributions including the envelope of the absorption spectrum, they disagree with the experiment in that the resonance widths are about one order of magnitude narrower than in the measured spectrum. Additional calculations for which the torsional angle of NO with respect to the intermolecular dissociation vector R is approximately taken into account as a fourth coordinate reveals that dephasing by out-of-plane motion can explain most of this discrepancy.