Stratification by study design showed HCC group had significantly higher leptin levels than CFC group in both case-control studies (SMD = 3.81, 95% CI (1.83, 5.79), P = 0.000) and nested case-control studies (SMD = 0.14, 95% CI (0.01, 0.26), P = 0.035). This evidence concerns the gene LEP and hepatocellular carcinoma.