Hypergammaglobulinemia increases serum viscosity and is the most common cause of hyperviscosity syndrome. Monoclonal hypergammaglobulinemia resulting in hyperviscosity syndrome is seen in multiple myeloma and Waldenström's macroglobulinemia. The reasons for elevated viscosity are increased protein content and large molecular size, abnormal polymerization, and abnormal shape of immunoglobulin molecules. Other hematologic and metabolic abnormalities seen in patients with plasma cell dyscrasias also contribute to hyperviscosity. Symptomatic hyperviscosity is much more common in Waldenström's macroglobulinemia (10 to 30%) than it is in myeloma (2 to 6%). Symptoms of hyperviscosity usually appear when the normal serum viscosity of 1.4 to 1.8 cp reaches 4 to 5 cp, corresponding to a serum immunoglobulin M (IgM) level of at least 3 g/dL, an IgG level of 4 g/dL, and an IgA level of 6 g/dL. Symptoms of hyperviscosity include constitutional symptoms; bleeding; and ocular, neurological, and cardiovascular manifestations. Immediate therapy of symptomatic hyperviscosity is directed at reduction of blood viscosity by plasmapheresis to control symptoms. Long-term management is directed at control of the underlying disease to prevent production of the monoclonal protein. There may be a small proportion of individuals, usually old or with severely compromised performance status, who undergo plasma exchange as the sole symptomatic therapy of hyperviscosity secondary to plasma cell dyscrasia.