Density-functional theory has been applied to the study of the mechanism of the Diels-Alder reaction of butadiene and ethylene. Both synchronous concerted and two-step diradical mechanisms were studied at the Becke3LYP/6-31G* level. The lowest energy stepwise pathway has a free energy of activation 7.7 kcal/mol above that of the concerted path. Spin correction of the spin-contaminated diradical transition structure energy reduces this energy difference to 2.3 kcal/mol. A study of the H-2 potential energy surface suggests that the spin-projection procedure overcorrects the energies of diradical species; the diradical energies likely fall between the corrected and uncorrected values. Thus, the free energy of concert for the Diels-Alder reaction is predicted to be between 2.3 and 7.7 kcal/mol, in excellent agreement with thermochemical estimates. Energies of reaction and geometries of the reactants and product are in good agreement with available experimental results. Calculated secondary kinetic isotope effects agree well with experimental data on a related reaction, and support a concerted mechanism for the butadiene plus ethylene Diels-Alder reaction. The Becke3LYP DFT method is capable of relatively economical direct comparisons of concerted and stepwise mechanisms.