Thirty-six hospitalized Thais (21 males, 15 females, 9 to 63 years of age) with Opisthorchis viverrini infection were treated with mebendazole in dosages of 20 mg/kg body weight for 4 weeks and 30 mg/kg body weight for 3 or 4 weeks. The drug was found to be effective when given in dosages of 30 mg/kg body weight for 3 to 4 weeks only a few eggs being found in the stool of one of 27 persons at 3 or 4 weeks post-treatment. In follow-up examination at 6 months, only two of 15 persons available were positive. These could have been reinfection, however. Side effects were essentially absent, except one person who vomited on the first day of treatment. The drug could be used in a mass treatment campaign because of effectiveness, low toxicity, and action against other helminthic infections.