  We analyze single transverse-spin asymmetries for hadronic pion production at
large transverse momenta using QCD factorization. In the large $x_F$ region,
leading contributions to the asymmetries are naturally produced by twist-3
parton correlation functions that couple quark fields and gluon field
strengths. With a simple model for these matrix elements, leading-order
asymmetries calculated from QCD are consistent with data on pion production
from Fermilab, and can be used to predict single-spin asymmetries at RHIC. We
argue that our perturbative calculation for the asymmetries is relevant to pion
transverse momenta as low as a few GeV.
