Background: Blood hyperviscosity has been acknowledged to be a complicating factor in polycythaemia and hyperproteinaemia. Hyperleucocytosis has also been implicated in hyperviscosity and may be a basis for therapeutic leukapheresis. Aims: This issue in the series seeks to determine the association and correlation between whole blood viscosity and white blood cell count (WBCC), with a view to advance the cause of a neglected clinical pathology index. Materials & Methods: Based on archived clinical pathology data, 10,857 cases that were concomitantly tested for full blood count and total proteins in the 2008 calendar year were audited for hyperleucocytosis. Whole blood viscosity level was determined and compared in the group with leucocytosis relative to the group with leucopenia and normal WBCC. The confounding effects of age, gender and red blood cell indices were evaluated. The correlation between whole blood viscosity and WBCC was also determined. Results: As a generalization, hypoviscosity is observed among individuals who presented hyperleucocytosis. There is no correlation (r = 0.20) between leucocytosis and blood viscosity. Conclusion: It is known that anaemia and thromboembolism, which can be associated with leucocytosis, predispose to hypoviscosity. The finding from this study provides evidence of association between hypoviscosity and leucocytosis. The absence of association and insignificant correlation between leucocytosis and hyperviscosity may be one explanation for ineffectiveness of therapeutic leukapheresis. Further, the non-correlation lends credence to specificity of blood viscosity, for which critical leucocytosis is not a substitute.