A blinded, randomized placebo-controlled trial assessed the efficacy and safety of oxfendazole for the treatment of ovine hydatid disease. Cyst fertility and parasite viability were measured following daily, weekly, and monthly treatment schedules with 30 mg of oxfendazole per kg of body weight. The 12-week trial was conducted in 215 adult sheep in the central Peruvian Andes and was masked for both treatment group and scheduling. In this trial oxfendazole significantly reduced protoscolex viability relative to controls in all treatment groups. In the daily, weekly, and monthly groups, 100, 97, and 78% of sheep, respectively, were either cured or improved following treatment, compared to 35% cured or improved animals in the control group. However, daily dosing at 30 mg of oxfendazole per kg proved highly toxic to sheep, resulting in a 24% death rate in the daily group as compared to a 4 to 6% mortality rate in all other groups. If found safe in humans, oxfendazole may prove to be a useful and inexpensive treatment for cestode infections in humans. This study suggests that a staggered dosing regimen of oxfendazole, and possibly other benzimidazoles, may be as efficacious as daily treatment regimens for hydatidosis while decreasing both the cost and adverse effects associated with daily dosing.