The ring-opening of cyclobutene (a) has been studied by methods based on density functional theory (DFT). Both the simple local spin density approximation (LDA) and a more extensive non-local (NL) approach has been applied. A detailed analysis of the atomic movements in the ring-opening process was further obtained by intrinsic reaction coordinate (IRC) calculations. The IRC study indicates that the ring-opening is a concerted process in which the rotation of the CH2 groups and the breaking of the C-C s-bond take place synchronously. Attention has also been given to the number of conformers for the 1,3-butadiene ring-opening product. The species s-trans-1,3-butadiene (e) of C2h symmetry was calculated to be the most stable conformer. The only other conformer calculated to be stable was a non-planar cis-1,3-butadiene species (c) of C2 symmetry. Planar s-cis-1,3 butadiene was found to be a transition state connecting the inter conversion of (c) to its enantiomer. Structures, relative energies and frequencies are provided for the species (a), (c), (d) and (e) as well as the ring-opening transition state (b). The DFT estimates were further compared with results from ab initio calculations on the species (a) through (e). The ab initio calculations were based on up to fourth order Møller-Plesset perturbation theory (MP4) and large basis sets of 6-311G** and 6-311+G** quality. It was found that the DFT methods provide as accurate estimates as the high level ab initio calculations of the properties for (a) through (e). The DFT frequencies compare better with the experimental values than the MP2 figures. The LDA method affords a good estimate of the barrier height for the forward reaction (a) Æ (c). However it underestimates the heat of reaction by as much as 5 kcal/mol. The NL corrections remove this error, and for the overall relative energies, the NL results are comparable to the MP4 estimates.