It is known that any symmetric matrix $M$ with entries in $\R[x]$ and which
is positive semi-definite for any substitution of $x\in\R$, has a Smith normal
form whose diagonal coefficients are constant sign polynomials in $\R[x]$. We
generalize this result by considering a symmetric matrix $M$ with entries in a
formally real principal domain $A$, we assume that $M$ is positive
semi-definite for any ordering on $A$ and, under one additionnal hypothesis
concerning non-real primes, we show that the Smith normal of $M$ is positive,
up to association. Counterexamples are given when this last hypothesis is not
satisfied. We give also a partial extension of our results to the case of
Dedekind domains.