  A theoretical study of the intense-field multiphoton ionization of
hydrogenlike systems is performed by solving the time-dependent Dirac equation
within the dipole approximation. It is shown that the velocity-gauge results
agree to the ones in length gauge only if the negative-energy states are
included in the time propagation. On the other hand, for the considered laser
parameters, no significant difference is found in length gauge if the
negative-energy states are included or not. Within the adopted dipole
approximation the main relativistic effect is the shift of the ionization
potential. A simple scaling procedure is proposed to account for this effect.
