Molecular orbital calculations using Hartree-Fock (HF), complete active space self consistent field (CASSCF), quadratic configuration interaction with single and double excitations, and triple excitations included via perturbation (QCISD(T)) and semiempirical (AM1) methods suggest that the experimentally observed endo selectivity for the Diels-Alder reaction between cyclopropene, I, and isotopically substituted butadiene, II, be mostly due to an attractive interaction between a C-H of I and the pi-bond between the central carbons of II, which is possible in the endo (but not exo) transition state. The relevance of this observation to other Diels-Alder endo/exo selectivities is discussed. The activation energetics for the reactions are ail substantially overestimated by HF and CASSCF calculations, while QCISD(T) and AM1 give more reasonable results. Only AM1, however, fails to predict the observed preference for the endo TS. Both QCISD(T) and small basis set calculations are shown to result in large basis set superposition errors, which fortuitously lower the (uncorrected) activation energies.