The hyperviscosity syndrome occurs when either abnormal amounts of serum protein are produced (multiple myeloma or Waldenström macroglobulinemia) or from the presence of massive numbers of white blood cells (chronic myelocytic leukemia [CML] and blast crisis). Vascular stasis and capillary sludging result in headache, fatigue, lethargy, seizures, and coma. Laboratory abnormalities in the paraproteinemias include RBC rouleaux formation, factitious hyponatremia, and abnormally high serum viscosity. Protein electrophoresis is diagnostic. Treatment of the hyperviscosity syndromes include hydration and plasmapheresis. Blast crisis and hyperviscosity in CML are treated with hydration and chemotherapy. Multiple myeloma is a common cause of destructive rib lesions and generalized osteopenia in patients older than 40 years. Additional clinical features include back pain (from lumbar vertebral fractures), constipation, and a decreased mental status (from hypercalcemia).