Mebendazole (R 17635) was tested in the treatment of mixed helminthiases in 140 male and female patients aged 4 to 14 years. These patients were necessarily treated by physicians visiting their restricted communities. Approximately 70% of the patients were infected by at least three, and the rest by only two, of the following helminths: hookworm, Ascaris lumbricoides, Enterobius vermicularis, Taenia sp, and Trichuris trichiura. Mebendazole (R 17635) was given in lOOmg tablets, one 30 minutes before breakfast and another three hours after dinner, for three consecutive days, independently of body weight. No evidence was observed of either immediate or late side effects of the ãrug. To determine parasitological cure, the Willis anã the Hoffman-Pons & Janer techniques were performed on the 7th, 14th, and 21st days after completion of treatment. In addition, in patients with taeniasis and/or enterobiasis, the "anal-swab" method was used for 7 consecutive days, beginning the 7th day after completion of treatment. Cure rates were as follows: enterobiasis, 100%; ascariasis and trichuriasis, 98%; hookworm, 94.5%. Out of 9 patients with taeniasis, 8 were cured; nevertheless, the Authors stress the necessity of larger studies, both technically and statistically, to determine the true efficacy of this drug against Taenia sp.