  We apply the Leggett-Garg inequalities to the case of classical and quantum
unstable systems. For classical systems the two assumptions of macroscopic
realism and non-invasive measurements imply that the three-measurements string
$K_3$ is identically equal to one. Also for quantum mechanical systems for
which the two assumptions are not valid anymore we find that $K_3=1$ for purely
exponential decays. On the other hand, the necessary deviations at short and
long times predicted by quantum mechanics lead to values of $K_3 \neq 1$. Thus,
we conclude that experiments in which such deviations have been observed should
also find in their data violations of the Leggett-Garg inequalities.
