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.