Background/Aims: Metabolic syndrome, comprising dia￢betes, hypertension, central obesity, and dyslipidemia, is increasingly prevalent worldwide. We aimed to study the re￢lationship between metabolic syndrome and the risk of liver fibrosis in patients with chronic hepatitis B (CHB) and chronic hepatitis C (CHC). Methods: In total, 954 patients (CHB, 850; CHC, 104 patients) with liver biopsy were included in the retrospective analysis. Extensive clinical and histologi￢cal data were available. Metabolic syndrome was defined using the International Diabetes Federation definition of metabolic syndrome, 2006 criteria. Histological lesions were evaluated according to the histology activity index system. Results: Metabolic syndrome was present in 6% of patients and significantly more prevalent in patients with CHC than in patients with CHB (5% vs 13%, p<0.001). Patients with met￢abolic syndrome were older among patients with CHB and patients with CHC, and, as expected, were mainly overweight or obese. Fibrosis was significantly more severe in patients with metabolic syndrome than in those without, regardless of whether they had CHB and CHC (CHB, 3.3±2.1 vs 2.4± 1.3, p=0.025; CHC, 2.6±1.5 vs 1.3±0.7, p=0.006). Liver fibrosis (stages 3 to 4) was independently associated with increased age, higher transaminase level and metabolic syn￢drome (odds ratio, 2.421; p=0.017). Conclusions: Metabolic syndrome is associated independently with severe fibrosis in patients with chronic viral hepatitis B and C.