  We present an extension of the Keldysh-Green's function method, which allows
to calculate the full distribution of transmitted particles through a
mesoscopic superconductor. The method is applied to the statistics of
supercurrent in short contacts. If the current is carried by Andreev bound
states the distribution corresponds to switching between long trains of
electrons going in opposite directions. For weak (gapless) superconductors or
tunnel junctions we find that at low temperatures the distribution has negative
`probabilities'. Accounting for the quantum mechanical nature of the measuring
device shows that these negative values can indeed be measured.
