A population-based study of the clinical, laboratory and ultrasonographic findings in patients suffering from mild or moderate opisthorchiasis in Prachinburi province, Thailand was conducted in 1990-1992. The effectiveness of treatment with praziquantel at 40 mg/kg body weight was evaluated. After treatment, a long-lasting, marked improvement in the well-being of the study group was observed. Symptoms common in opisthorchiasis infection decreased in intensity and the clinical response showed total or partial remission in 98% of all cases studied. Total and direct bilirubin concentrations decreased significantly and remained low up to the end of the follow-up period of 2 years, indicating a reduction in cholestasis. Also, white blood cell counts decreased initially, which can be interpreted as a reduction in inflammation intensity. No relationship was found between intensity of infection and age or clinical findings. Population-based treatment of opisthorchiasis appears to have had a significant impact on public health in north-east Thailand. However, it is also evident that drug therapy alone will not solve the opisthorchiasis problem, as indicated by the reinfection rate of almost 10% at the end of the study.