Summary Bithionol in daily doses of 20 to 50 mg/kg given orally, on alternate days, was used in the treatment of 80 patients with paragonimiasis and results were followed for 6 months to 1 year. A daily dosage of 30 to 40 mg/kg given 10 times produced a satisfactory effect. For children the recommended daily dosage, given divided with meals, is 1.5–2.0 g for ages 12 years and over, 1.0 g for ages 9–12, 0.6 g for ages 5–8, and 0.2 g for ages less than 5 years. Among 80 cases, there were two apparent relapses—in one the amount of drug received was 20 mg/kg 10 times and in the other, 40 mg/kg 5 times. Relapse was not observed in patients receiving 10 or more doses of 30 mg/kg. In one case 5 doses of 50 mg/kg gave complete cure, but the advisability of further reducing the number of doses is uncertain. As side-effects are minimal with administration on alternate days, extension of the term of treatment appears to be unnecessary. After two to five doses of bithionol, no Paragonimus eggs appeared in bloody sputum or in feces, and eosinophilia decreased to normal soon after the completion of treatment. Diarrhea, abdominal pain, nausea, vomiting and urticarial eruption of dermatitis were observed as transient side-effects, less frequently in children than in adults. About 20 percent of lesions apparent as chest X-ray shadows due to paragonimiasis were absorbed immediately after treatment, most were absorbed in 1 to 3 months and only a few remained unchanged for longer than 6 months to 1 year. It may be possible to decide the effect of treatment comparatively early by observing roentgenographic changes in lungs. Bithionol was not effective in one observed case of clonorchiasis.