BACKGROUND Although hyperviscosity syndrome in plasma cell dyscrasia (PCD) and thrombosis in myeloproliferative neoplasm (MPN) are major causes of morbidity and mortality, blood viscosity measurements are often underutilized. OBJECTIVE This study aimed to characterize whether whole blood viscosity (WBV) or plasma viscosity (PV) could be predictive of hyperviscosity syndrome in PCD and could be elevated in subgroups of MPN. METHODS A total of 75 patients with hematologic diseases: PCD (n = 26), MPN (n = 25) including polycythemia vera (P. vera) and lymphoma (n = 24) were enrolled along with 104 healthy controls. Both WBV and PV were measured using a capillary tube viscometer. Hyperviscosity syndrome was defined as having 2 or more hyperviscosity symptoms. RESULTS Patients with PCD showed significantly higher PVs at high and low shear rates when compared to healthy controls, especially in those with hyperviscosity syndrome. The sensitivity and specificity of WBV and PV in detecting hyperviscosity syndrome were 28.6% and 94.1%, and 71.4% and 66.7%, respectively. Patients with P. vera exhibited high WBV and RBC counts compared to healthy controls. CONCLUSIONS PV is predictive of hyperviscosity syndrome in PCD and WBV is elevated in patients with P. vera. It suggests that hemorheologic disturbances exist in patients with PCD and MPN and that tests of viscosity may be helpful in detecting hemorheological disturbances.