We found, by using the theoretical MP2 model, that correlation energy of the valence electrons in a large number of 3D organic molecules follows a very simple additivity rule resembling that established earlier in planar pi-systems. Namely, it turns out that the total correlation energy is a multilinear function of the number of atoms of each element entering a molecule. Extrapolating the calculated correlation energies to the complete basis set (CBS) values, it occurs that the additivity holds for E(CBS)(corr) too. We believe that computational methods more rigorous than MP2 will confirm the additivity in the future and show that it is a genuine molecular property. The additivity formula for the valence electrons correlation energies could serve as a diagnostic tool to identify cases where significant nonadditivity takes place. In such situations the electronic structure apparently exhibits some subtleties, which are not present in other, more or less related molecules, thus deserving a meticulous scrutiny. The additivity of the Hartree-Fock energies was examined too, Deviations from the additivity in the selected set of gauge molecules (substituted alkanes) is much higher than for the correlation energy. Highly strained molecules exhibit dramatic nonadditivities, which are identified as-the angular strain energies. It is found that HF energies extrapolated to the complete basis yield the angular strain destabilizations much closer to the experimental estimates. Introduction of the offset value enables almost quantitative prediction of the angular strain effect in the series cyclopropane, cyclobutane, cyclopentane, and tetrahedrane.