A high prevalence of the liver flukes Clonorchis sinensis and Opisthorchis viverrini has been reported in a number of provinces in Vietnam. Knowledge about C. sinensis infection gained from Thailand over the past decade suggests that the habit of eating raw freshwater fish is a major risk factor for infection. However, further information to confirm this is needed. In the present study 1155 villagers in two communes in northern Vietnam were interviewed and their stools were examined for the presence of liver flukes. The prevalence of infection was 26% and was 3.6 times higher in males than in females. The habit of eating raw fish increased the risk of C. sinensis infection 53-fold. These results provide evidence of a strong correlation between the intensity of C. sinensis infection and the cumulative quantity of freshwater fish consumed in a lifetime and suggest that simple questionnaires could be used in endemic areas to quickly identify populations at risk and enable targeted treatment.