In this paper the unitary equivalence of unbounded
*-representations of *-algebras
is investigated. It is shown that if closed *-representations
π1 and π2 of a *-algebra
[Ascr ] satisfy a certain density condition for the intertwining spaces
[Jscr ](π1, π2) and
[Jscr ](π2, π1), then a *-isomorphism
Φ between the
O*-algebras π1([Ascr ]) and π2([Ascr ])
is
defined by Φ(π1(x))=π2(x),
x∈[Ascr ] and it induces a
*-isomorphism Φ¯, between the von Neumann algebras
(π1([Ascr ])′w)′
and (π2([Ascr ])′w)′,
and
further if Φ¯, is spatial (that is, it is
unitarily implemented), then π1 and π2
are unitarily equivalent.