  In this paper we generalize Brown's spectral distribution measure to a large
class of unbounded operators affiliated with a finite von Neumann algebra.
Moreover, we compute the Brown measure of all unbounded R-diagonal operators in
this class. As a particular case, we determine the Brown measure of z=xy^{-1},
where (x,y) is a circular system in the sense of Voiculescu, and we prove that
for all positive integers n, z^n is in L^p(M) iff 0<p< 2/(n+1).
