  The non--commuting graph $\Gamma(G)$ of a non--abelian group $G$ is defined
as follows. The vertex set $V(\Gamma(G))$ of $\Gamma(G)$ is $G\setminus Z(G)$
where $Z(G)$ denotes the center of $G$ and two vertices $x$ and $y$ are
adjacent if and only if $xy\neq yx$. For non--abelian finite groups $G$ and $H$
it is conjectured that if $\Gamma(G) \cong \Gamma(H)$, then $|G|=|H|$. We prove
the conjecture.
