DFT calculations indicate a surprisingly strong cis orientational preference of the two NO ligands of dinitrosylheme, a unique reactive species of potential biological relevance that has been recently generated and spectroscopically characterized at low temperature. While a trans centrosymmetric conformation has been proposed on the basis of IR spectroscopic data, DFT calculations favor a cis C2v conformation of the two NOs over a trans orientation by about 20 kcal/mol.