Potential risk factors for cholangiocarcinoma were investigated in a case-control study among inhabitants of north-east Thailand, which included 103 cases from 3 hospitals, with age-and sex-matched controls. A clear association with past or present infection with Opisthorchis viverrini, as indicated by raised serum antibodies, was found (o.r. 5.0), and at least two-thirds of cases can be attributed to this cause. The results suggest that males may be at higher risk than females. There was no association with hepatitis B infection, with aflatoxin Intake as estimated from albumin adducts in serum or with any particular dietary patterns. Alcohol consumption was very low in the population, and the risk associated with regular drinking was non-significant. Regular users of betel nut—predominantly female—had a high risk (o.r. 6.4), a possible mechanism being through their increased exposure to nitro-samines.