Hepatocellular carcinoma (HCC) from regions with high dietary exposure to aflatoxins and endemic for hepatitis B virus (HBV) often contain a specific mutation at codon 249 in TP53 (249 ser ; AGG to AGT, Arg to Ser). This mutation is also detectable in circulating cell-free DNA from the plasma of HCC patients and healthy subjects in these regions. We have examined the joint effect of plasma 249 ser and HBV infection in a case-control study design involving 348 control, 98 cirrhotic, and 186 HCC participants from The Gambia, West Africa, an area of high HCC incidence. The 249 ser mutation was detected in 3.5% of controls, 15.3% of cirrhotics, and 39.8% of HCC cases (adjusted odds ratios (OR): 4.83, (95% confidence interval (CI): 1.71-13.7) for cirrhosis and 20.3 (8.19-50.0) for HCC). HBsAg positivity along with plasma 249 ser was observed in 45/183 (24.6%) HCC cases compared to only one (0.3%) control. Risk for HCC was associated with markers of HBV alone (OR: 10.0, 95% CI: 5.16-19.6), 249 ser alone (OR: 13.2, 95% CI: 4.99-35.0), and both markers present (OR: 399, 95% CI: 48.6-3270). These results suggest a multiplicative effect on HCC risk resulting from the mutational effect of aflatoxin on TP53, as monitored by detection of plasma 249 ser , with concomitant chronic infection with HBV.