The clinical course of two patients who presented with polyclonal hyperviscosity syndrome is described. Polymerizing IgG rheumatoid factors were isolated from the serum of both patients. The presence of these polymers with their extraordinary rheologic properties was the principal determinant of the abnormal rheological properties of the plasma. Neither patient had a discrete rheumatic disease. In both, a lymphoproliferative syndrome was present with features suggestive of the "pseudolymphoma" of Sjögren's syndrome.