  Two--photon decay of hydrogen--like ions is studied within the framework of
second--order perturbation theory, based on relativistic Dirac's equation.
Special attention is paid to the effects arising from the summation over the
negative--energy (intermediate virtual) states that occurs in such a framework.
In order to investigate the role of these states, detailed calculations have
been carried out for the $2s_{1/2} - 1s_{1/2}$ and $2p_{1/2} - 1s_{1/2}$
transitions in neutral hydrogen H as well as for hydrogen--like xenon
Xe$^{53+}$ and uranium U$^{91+}$ ions. We found that for a correct evaluation
of the total and energy--differential decay rates, summation over the
negative--energy part of Dirac's spectrum should be properly taken into account
both for high--$Z$ and low--$Z$ atomic systems.
