One hundred fifteen patients with light to severe infections of Trichuris trichiura were treated with mebendazole. A double blind study was designed with test subjects receiving placebos, and 2, 3 and 4 days of active medication, 100 mg twice daily. Extensive blood and urine analyses before, during, and 8 days after initial treatment showed no significant changes due to the administration of mebendazole. Patients tolerated the drug well and no toxic side effects were observed. Egg count comparisons on stools collected before and after treatment indicated cure rates of 53.3%, 64.3% and 66.7% for patients on the 2, 3 and 4 day regimens respectively, after 42 days. In the patients not cleared of their worm burdens, an 87.3% to 96.1% egg reduction was observed.