Flavin cofactors are essential for the biological function of many electron-transfer proteins. The electron spin density distribution in the semiquinone (radical) state of the flavin ring has been calculated using the B3LYP hybrid functional in combination with the EPR-II basis set. Both the isolated flavin and the flavin surrounded by small molecules that mimic the environment found in flavoproteins have been analyzed. The validity of the results has been checked by comparison with experimental hyperfine coupling parameters previously reported. The effects of the flavin/protein interaction on the flavin spin density distribution have been discussed. A peculiar behavior of the spin density in some atoms of the flavin ring is found that could be relevant in understanding reaction mechanisms in flavoproteins.