Upper bound on the hadronic light-by-light contribution to the muon g-2

There are indications that hadronic loops in some electroweak observables are almost saturated by parton level effects. Taking this as the hypothesis for this work, we propose a genuine parton level estimate of the hadronic light-by-light contribution to the anomalous magnetic moment of the muon, a(mu)(LBL) (had). For infinitely heavy quarks our treatment is exact, while for asymptotically small quark masses a(mu)(LBL) (had) is overestimated. Interpolating, this suggests to quote an upper bound. We obtain a(mu)(LBL) (had) < 1.59 x 10(-9) (95% CL).