A total of 131 hospitalized patients with Capillaria philippinensis infections were given mebendazole in doses ranging from 100 mg to 400 mg daily in single and divided doses for 10 to 30 days. Nearly all patients showed clinical improvement without side effects, but treatment failures occurred in a number of patients given 100 to 300 mg of the drug daily. No treatment failure occurred in new cases treated with 400 mg daily in divided doses for 20 days, but three failures occurred in old relapsed or chronic cases given the same dose for 30 days. However, the latter were successfully retreated with the same schedule. Mebendazole is presently considered the anthelminthic of choice for the treatment of this disease.