We report new statistical
equilibrium calculations for Fe I and Fe II in the atmosphere of late-type
stars. We used atomic models for Fe I and Fe II having, respectively, 256 and
190 levels, as well as 2117 and 3443
radiative transitions. Photoionization cross-sections are from the Iron Project.
These atomic models were used to investigate non-LTE (NLTE) effects in iron
abundances of late-type stars with different atmospheric parameters. We found
that most Fe I lines in metal-poor stars are formed in conditions far from LTE.
We derived metallicity corrections of about 0.3 dex with respect to LTE values,
for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to be affected by
significant NLTE effects. The main NLTE effect invoked in the case of Fe I is overionization
by ultraviolet radiation, thus classical ionization equilibrium is far from
being satisfied. An important consequence is that surface gravities derived by
LTE analysis are in error and should be corrected before final abundances
corrections. This apparently solves the observed discrepancy between spectroscopic
surface gravities derived by LTE analyses and those derived from Hipparcos
parallaxes. A table of NLTE [Fe/H] and log g values for a sample of metal-poor
late-type stars is given.