Bithionol—2,2′thiobis(4,6-bichlorphenol)—was used for the mass treatment of 1,355 patients with proved infections with Paragonimus westermani. The dosage used was 40 mg per kg of body weight per day, on alternate days, for either 10 or 15 doses, but some patients took only 5 to 9 doses. During treatment, acute side effects occurred in 319 (41.2%) of 775 patients taken at random for questioning. One year after treatment, sputum from 1,166 patients was examined for eggs of P. westermani. No eggs were found in 73.6% of 53 patients who received less than 10 days of therapy, nor in 90.8% of 1,113 patients who took the longer course.