  We give a simple proof of the relation $\Lambda\p_artial{\Lambda}\F=
{i\over2\pi}b_1\langle\Tr\phi^2\rangle$, which is valid for $N=2$
supersymmetric QCD with massless quarks. We consider $SU(N_c)$ gauge theories
as well as $SO(N_c)$ and $SP(N_c)$. Aa analogous relation which corresponds to
massive hypermultiplets is written down. We also discuss the generalizations to
$N=1$ models in the Coulomb phase.
