  We calculate the cross section for the inclusive production of single hadrons
with finite transverse momenta in deep-inelastic scattering at next-to-leading
order (NLO), i.e. through O(alpha_s^2), in the parton model of QCD endowed with
non-perturbative parton distribution functions (PDFs) and fragmentation
functions (FFs). The NLO correction is found to produce a sizeable enhancement
in cross section, of up to one order of magnitude, bringing the theoretical
prediction to good agreement with recent measurements for neutral pions and
charged hadrons at DESY HERA. This provides a useful test for the universality
and the scaling violations of the FFs predicted by the factorization theorem.
Such comparisons can also be used to constrain the gluon PDF of the proton.
