Ab initio (SCF, MP2, MP3, and CCSD(T)) and DFT (B3LYP) calculations were done on a variety of five-membered cyclic dienes, (CH)(4)X (X = CH2, SiH2, O, NH, PH, and S) with ethylene as a dienophile. Comparison of results with the available experimental data indicates that the CCSD(T) level with the 6-31G* basis set is adequate in obtaining quantitative answers, both in terms of activation barriers and reaction energies. The hybrid density functional B3LYP and ab initio MP3 methods showed very good agreement with CCSD(T) results. The HF and MP2 methods are good only in reproducing the trends and fail in quantitative agreement. The changes as a function of basis set in energetics are rather small at B3LYP and MP3 levels, thus indicating that the 6-31G* basis set is adequate to model Diels-Alder reactions'. However, the dynamic electron correlation is essential in obtaining correct activation energies. The present systematic study indicates that MP3/6-31G* is the minimum necessary level of theory to obtain reliable predictions both in terms of activation energies and in terms of reaction energies. The thermochemical data obtained at the HF method is essentially identical to that obtained at B3LYP level.