  Recently, it has been shown, contrary to previous beliefs, that the k_T
distribution of quarks in a transversely polarized proton can be asymmetric.
This ``Sivers effect'' had already been used to explain transverse single spin
asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p
--> pion + X and p-bar(transv. polarized) p --> pion + X. In such channels,
however, other mechanisms, like the ``Collins effect'' (a k_T asymmetric
fragmentation of a transversely polarized quark into pions), may generate SSA.
The Sivers asymmetry is used here to compute SSA in Drell-Yan processes; in
this case, by considering the differential cross-section in the lepton-pair
invariant mass, rapidity and transverse momentum, other mechanisms which may
originate SSA cannot contribute. Estimates for RHIC experiments are given.
