  Real sources of entangled photon pairs (like parametric down conversion) are
not perfect. They produce quantum states that contain more than only one photon
pair with some probability. In this paper it is discussed what happens if such
states are used for the purpose of quantum key distribution. It is shown that
the presence of "multi-pair" signals (together with low detection efficiencies)
causes errors in transmission even if there is no eavesdropper. Moreover, it is
shown that even the eavesdropping, that draws information only from these
"multi-pair" signals, increases the error rate. Information, that can be
obtained by an eavesdropper from these signals, is calculated.
