Hemorheologic changes from silent to acute and chronic cerebral infarction have seldom been reported. We evaluated hemorheologic profiles of the whole blood viscosity, plasma viscosity and fibrinogen level in stroke at-risk patients with silent cerebral infarction, patients with acute or chronic cerebral lacunar infarction, and subjects at low risk for stroke. Hemorheologic profiles were measured in 88 subjects: (1) 36 patients with silent cerebral infarction (mean 64.7 years), who provided no clinical history of having had definitive stroke but showing > 5 mm lesions of cerebral infarction or periventricular hyperintensity (PVH) observed in magnetic resonance imaging (MRI) T2-weighted images; (2) 12 patients with acute cerebral lacunar infarction (mean 69.1 years), measured within 3 days and repeated 1 month after onset; (3) 25 patients with chronic cerebral lacunar infarction (mean 66.2 years), measured 12.5 months after onset; and (4) 15 subjects at low risk for stroke (mean 65.8 years) without cardiovascular risk factors or lesions on MRI. Patients with silent cerebral infarction were subdivided into two groups of less advanced and more advanced grades, based on the number of infarctions or the grade of PVH. Whole blood viscosity (shear rates: 22.5-225.0 s-1), corrected blood viscosity for 45% standard hematocrit (Hct), plasma viscosity, fibrinogen, serum total protein, albumin, and Hct were measured. Plasma fibrinogen levels were lower in silent cerebral infarctions than in chronic cerebral infarctions (P < 0.01), and patients with more advanced grades of silent cerebral infarction showed higher levels of plasma fibrinogen than those with less advanced grades (P < 0.01 and P < 0.05). Whole blood viscosity, corrected blood viscosity (Hct 45%), plasma viscosity and fibrinogen levels in acute cerebral infarction within 3 days after onset were higher significantly than those in subjects at low risk for stroke. Plasma fibrinogen level persisted to be elevated up to 1 month after onset, which continued as well in patients with chronic cerebral infarction. Advanced grades of silent cerebral infarction in stroke at-risk patients are accompanied by elevations of plasma fibrinogen level, which increases further after onset of cerebral infarction; such abnormalities persist up to the chronic stage. Elevated plasma fibrinogen level might reflect progression of atherogenesis in patients with advanced grades of silent cerebral infarction, resulted in an increased probability as to be a risk factor for cerebral infarction.