  We name an indecomposable symmetrizable generalized Cartan matrix $A$ and the
corresponding Kac--Moody Lie algebra ${\goth g} ^\prime (A)$ {\it of the
arithmetic type} if for any $\beta \in Q$ with $(\beta | \beta)<0$ there exist
$n(\beta )\in {\Bbb N}$ and an imaginary root $\alpha \in \Delta^{im}$ such
that $n(\beta )\beta \equiv \alpha \mod Ker\ (.|.)$ on $Q$. Here $Q$ is the
root lattice. This generalizes "symmetrizable hyperbolic" type of Kac and
Moody. We show that generalized Cartan matrices of the arithmetic type are
divided in $4$ types: (a) finite, (b) affine, (c) rank two, and (d) arithmetic
hyperbolic type. The last type is very closely related with arithmetic groups
generated by reflections in hyperbolic spaces with the field of definition
$\Bbb Q$. We apply results of the author and \'E.B. Vinberg on arithmetic
groups generated by reflections in hyperbolic spaces to describe generalized
Cartan matrices of the arithmetic hyperbolic type and to show that there exists
a finite set of series of the generalized Cartan matrices of the arithmetic
hyperbolic type. For the symmetric case all these series are known.
