Anticardiolipin antibody (aCL) is considered to be one of the contributory factors in the development of acute ischemic stroke. Chronic hepatitis C virus infection (HCV) and the antiphospholipid syndrome are two conditions that have increased the risk of stroke. The purpose of this study was to investigate the prevalence of anticardiolipin antibodies in patients with asymptomatic chronic hepatitis C virus infection-related acute ischemic stroke. Fifty eight patients (39 women, 19 men), mean age 62 years (range 46-77 years) with acute ischemic stroke and asymptomatic chronic hepatitis C virus infection were studied; all were positive for anti-HCV antibodies. A control group of 36 patients (20 women, 16 men), mean age 58 years (range 44-75 years) with ischemic stroke without HCV were also tested. Both anti-HCV antibodies and IgG aCL antibodies were measured by enzyme-linked immunosorbent assay (ELISA). The significantly higher levels of IgG aCL antibodies were detected in serum samples of 27/58 (46%) patients with acute ischemic stroke and asymptomatic chronic hepatitis C virus infection. The control group had much lower levels of IgG aCL antibodies in 2/36 (5%) patients. This study clearly shows a high prevalence of IgG aCL antibodies in the serum of patients with acute ischemic stroke and HCV. Mean levels of serum IgG aCL antibodies were significantly higher among the HCV patients with acute ischemic stroke than the patients with acute ischemic stroke without HCV (94.8+/-7.6 GPL U/mL as compared to 27.2+/-3.2 GPL U/mL), p<0.001. We showed that a number of chronic hepatitis C virus infection-related ischemic strokes is associated with the presence of anticardiolipin antibodies, a condition predisposing to coagulopathy. These results suggest that anticardiolipin antibodies associated with HCV may be an important marker for acute ischemic stroke. The higher prevalence and titre of IgG aCL antibodies in patients with HCV suggests that these humoral factors might be involved in the pathogenesis of ischemic stroke.