Hyperviscosity syndrome (HVS) refers to the clinical sequelae of increased blood viscosity. Increased serum viscosity usually results from increased circulating serum immunoglobulins and can be seen in such diseases as Waldenstr?m macroglobulinemia and multiple myeloma. HVS can also result from increased cellular blood components in hyperproliferative states such as the leukemias, polycythemia, and the myeloproliferative disorders. The clinical presentation in HVS consists principally of the triad of mucosal bleeding, visual changes, and neurologic symptoms. Constitutional symptoms and cardiorespiratory symptoms may also occur.