Twenty-three equine sarcoids in 22 horses were treated, using an afterloading technique, with iridium-192 (192Ir) interstitial radiation therapy. Previously unsuccessful treatment in 14 horses included surgical excision, cryosurgery, hyperthermia, and nonspecific immunotherapy. Twenty tumors were located in the skin of the head, and three were in the skin of the extremities. Before implantation, surgical debulking was done in six tumors, and hyperthermia (43°C, 30 min) was done in three tumors. Total implantation time varied from 4 to 14 days to give radiation doses of 52–93 Gy. The criteria for assessment of therapy were tumor response, tumor-free incidence at one year, and complications. Follow-up times ranged from six to 41 months (mean, 20 months). All tumors of the head and two tumors of the extremities had complete response to therapy. One sarcoid of the extremity had partial response to treatment. Tumor recurence was seen in the head of one horse. Of 16 horses followed for one year, the tumor-free incidence was 94% (15/16). Complications were severe local necrosis in two horses and deep infection in one horse. It was concluded that 192Ir interstitial brachytherapy was an effective treatment of recurrent and difficult-to-manage equine sarcoid.