Background High Mobility Group Box-1 (HMGB-1) is implicated in the pathogenesis of thrombosis and cancer. In the present study, we aimed to evaluate its potential role as a diagnostic biomarker of portal vein thrombosis (PVT) among patients with hepatocellular carcinoma (HCC). Methods The study population included N=100 prospectively recruited patients with a novel diagnosis of HCC. We compared circulating HMGB-1 levels between 34 healthy controls, HCC patients with PVT (N=22), and HCC patients without PVT (N=78). Results HCC patients without PVT showed significantly higher median HMGB-1 serum levels (8.2 [5.5-13.1] ng/ml) than those observed in the case of HCC with PVT (5.5 [4.4-8.5] ng/ml; p=0.012) and in healthy controls (4.1 [3.1-8.2] ng/ml; p<0.001). Among HCC patients, at univariate analysis, the presence of PVT was associated with higher median age (p=0.036), larger major cancer node diameter (p<0.001), and lower HMGB-1 serum level (p=0.012). At multivariate analysis, the presence of PVT maintained a positive association with major node diameter p=0.001) and an inverse association with serum HMGB-1 levels (p=0.003). Conclusions These findings indicate that serum HMGB-1 bears an inverse association with PVT complicating HCC at the time of diagnosis and suggest that serum HMGB-1 depletion may mark PVT development in cirrhotic patients with HCC.