  We calculate the leading-order hadronic correction to the anomalous magnetic
moments of each of the three charged leptons in the Standard Model: the
electron, muon and tau. Working in two-flavor lattice QCD, we address
essentially all sources of systematic error: lattice artifacts, finite-size
effects, quark-mass extrapolation, momentum extrapolation and disconnected
diagrams. The most significant remaining systematic error, the exclusion of the
strange and charm quark contributions, will be addressed in our four-flavor
calculation. We achieve a statistical accuracy of 2% or better for the physical
values for each of the three leptons and the systematic errors are at most
comparable.
