  We consider the consequences of appreciable line optical depth for the
profile shape of X-ray emission lines formed in stellar winds. The hot gas is
thought to arise in distributed wind shocks, and the line formation is
predominantly via collisional excitation followed by radiative decay. Such
lines are often modelled as optically thin, but the theory has difficulty
matching resolved X-ray line profiles. We suggest that for strong lines of
abundant metals, newly created photons may undergo resonance scattering,
modifying the emergent profile. Using Sobolev theory in a spherically symmetric
wind, we show that thick-line resonance scattering leads to emission profiles
that still have blueshifted centroids like the thin lines, but which are
considerably less asymmetric in appearance. We focus on winds in the
constant-expansion domain, and derive an analytic form for the profile shape in
the limit of large line and photoabsorptive optical depths. Our theory is
applied to published {\it Chandra} observations of the O star $\zeta$ Pup.
