I just came back from a cancer conference and had a long conversation with a leading doc at MDACC. One of the things that came out of that conversations was that people with unknown primaries have a slight survival advantage in MDACC's clinical experience, (also reported at 3 other major centers). The thinking is just as posted above, that the body's immune system has in some manner dealt with the primary, which was likely very small, and all that is left is the node involvement. This was the first that I had heard of this, but for those thrown into the uncertainty generated by an unknown primary, perhaps it is of some comfort.