  We report on our studies of the renormalization of flavor singlet quark
bilinear operators in lattice QCD. The renormalization constants are determined
non-perturbatively using gauge field ensembles with Nf=2 dynamical clover
improved fermions. The renormalization is performed in the RI'-MOM scheme. The
difference between flavor singlet and non-singlet quark bilinear operators is a
disconnected contribution, which has to be evaluated by stochastic estimators.
We compare our results for the running with perturbation theory.
