Plasma and serum viscosity measurements have an important role in the clinical management of patients prone to hyperviscosity syndrome. Hyperviscosity syndrome can occur in association with a variety of clinical diagnoses, such as polycythemia, Waldenström macroglobulinemia, multiple myeloma, and leukemia. The viscosity at which symptoms develop is variable, but they rarely appear below a serum or plasma viscosity of 3 cP. Therefore, accurate measurement of serum or plasma viscosity is an important component of patient management.