One hundred thirty-two ambulatory patients, harboring 284 intestinal worm infections, were treated with thiabendazole. Dosage and administration of the drug were varied in four different groups studied. Best results were obtained against Strongyloides stercoralis, Ascaris lumbricoides and hookworms. Effectiveness was very weak for Trichuris trichiura, and no effect was found in Taenia solium or T. saginata infections. From one-third to one-half of the patients showed manifestations of intolerance, usually transient and not severe. A review of the publications on clinical trials up to September 1964 is presented.