The effects of three benzimidazole anthelmintics in dogs infected with a human strain of Strongyloides stercoralis were investigated. Cambendazole, but not thiabendazole or mebendazole, abrogated the subsequent development of a patent infection when administered at the same time as infection to immunocompetent dogs. None of the drugs eradicated infection when given after the onset of patency in immunosuppressed animals, although worm burdens were greatly reduced in dogs treated with cambendazole. The implications of these findings for the treatment of patients with strongyloidiasis, particularly those with disseminated infections, are discussed.