Mebendazole (methyl-5-benzoylbenzimidazole-2-carbamate), a new broad spectrum anthelmintic, cured 26 (74% out of 35 children with severe symptomatic trichuriasis. Repeat courses of therapy were necessary in some cases. The nine treatment failures all showed large reductions (82–99%) in egg counts. Thus, an overall satisfactory response was obtained in terms of complete parasite eradication in 74%, relief of symptoms in all, and significant reduction in worm load in the failures. In the recommended dosage of 100 mg twice daily for 3 days mebendazole failed to achieve a cure rate of 95% to 100% reported in asymptomatic individuals and those with light infections of Trichuris trichiura. It is probable that measures to control diarrhea before administering mebendazole in an increased, extended, or repeated dosage schedule will achieve a higher cure rate. The drug was well tolerated and completely free from any toxic effects.