We describe an innovative strategy to quantify risk of cancer associated with varying levels of exposure to chronic parasitic infection through the identification of asymptomatic cases of cholangiocarcinoma within a population-based survey of Opisthorchis viverrini infection. Stool samples from 12,311 adults over age 24 years from 85 villages in northeast Thailand were examined for intensity of liver fluke infection. People from varying egg count categories were selected for ultrasound examination to identify hepatobiliary disease. Fifteen preclinical cases of cholangiocarcinoma were diagnosed from a total of 1,807 people based on ultrasonographic evidence with confirmation by endoscopy where possible. The prevalence odds of the diagnosis of cholangiocarcinoma increased gradually within the light and moderate intensity groups. In contrast, sharply elevated prevalence odds [age-, sex- and locality-adjusted prevalence odds ratio (POR) 14.1, p < 0.05] were observed within the most heavily liver fluke-infected group compared with the uninfected group. Males were more frequently affected than females (crude POR 4.5), but after controlling for intensity of infection, age and locality, the magnitude and significance of this measurement was reduced. Our data clearly demonstrate a significant relationship between intensity of liver fluke infection and cholangiocarcinoma and a strikingly high prevalence of the disease among heavily infected males.