  We prove that for a Wightman quantum field $\phi(x)$ the assumptions (i)
positivity of the metric on the state space ${\cal H}$ of the theory (ii) the
asymptotic condition in the form of Yang-Feldman equations and (iii)
Klein-Gordon equation for the outgoing field imply that the states generated by
application of the asymptotic fields to the vacuum are stable. We prove by a
counter example that this statement is wrong in the case of quantum fields with
indefinite metric.
