Chronic inflammation and oxidative stress play pivotal roles in the pathogenesis of hepatocellular carcinoma (HCC). We conducted a nested case-control study of 347 HCC cases and 691 matched controls within a prospective cohort of 18 244 Chinese men in Shanghai, China. The concentrations of 8-epi-prostaglandin F2α (8-epi-PGF2α), a biomarker of oxidative stress, and prostaglandin E2 (PGE2) metabolite (PGE-M), a biomarker of the inflammation mediator PGE2, were determined in baseline urine samples using validated mass spectrometry assays. 8-epi-PGF2α levels were significantly higher in HCC cases than control subjects (geometric means 0.92 versus 0.80 pmol/mg creatinine, P < 0.001). The relative risks of developing HCC for the highest relative to the lowest quartile of 8-epi-PGF2α were 2.55 (95% confidence interval = 1.62-4.01, Ptrend < 0.001). This positive 8-epi-PGF2α-HCC risk association was independent of smoking status, alcohol consumption and hepatitis B or liver cirrhosis and was present 10 years before the clinical manifestation of HCC. This study did not find any significant association between urinary PEG-M and HCC risk. This study provides direct evidence in support of the critical role of oxidative stress in the development of HCC regardless of its underlying causes.