Hyperviscosity syndrome (HVS) refers to the clinical sequelae of increased blood viscosity. Blood viscosity is influenced by number of factors including hematocrit, plasma protein and fibrinogen concentrations, cellular aggregation, red cell deformability and axial migration, vessel diameter, and flow rate . The complicated physiopathological changes caused by HVS may lead to vascular thrombosis and stroke. Haematological disorders account for up to 8% of all ischemic strokes in different series. In this chapter we discuss the case of a 58-year-old male patient who presented with recurrent left hand weakness and dysarthria, no common cerebrovascular risk factors and suffered from nonspecific clinical manifestations including weight loss, fatigue, blurred vision and intermittent nosebleed for the past three months. The MRI demonstrated multifocal ischemic stroke of the right hemisphere; the standard stroke diagnostic work-up showed negative findings. Ultimately the patient was diagnosed with Waldenström macroglobulinemia (WM) by bone marrow biopsy. We hypothesized that in this patient hyperviscosity syndrome caused by WM may decreased blood flow and caused microcirculatory disorders, leading to the development of cerebral infarction