  The anti-kaon nucleon system close to threshold is analyzed in view of the
new accurate DEAR kaonic hydrogen data. The calculations are performed using
chiral SU(3) effective field theory in combination with non-perturbative
schemes based on coupled channels. Several variants of such approaches are
compared with experimental data and the differences in the results are
discussed. Coulomb and isospin breaking effects turn out to be important and
are both taken into account. The pole structure of the Lambda(1405) resonance
close to the anti-kaon nucleon threshold is critically examined.
