Twenty.four dogs with anal furunculosis were treated with cyclosporine once daily for 13 weeks at dosages of 1.5, 3.0, 5.0 or 7.5 mg/kg, and re.examined after six and 12 months. After 13 weeks the disease in six of the dogs was in remission, 11 were controlled or improved and seven had failed to respond. The response of the dogs given the highest dose was significantly better than the response of the other groups taken together (P<0.01 4), and better than the responses of the groups given 1.5 mg/kg and 5 mg/kg (P<0.05). The dogs improved clinically during the treatment, most rapidly during the first five weeks. Of the six dogs that were in remission after 13 weeks, three relapsed after one, two and six months. The 1 1 dogs that were improved or controlled after 13 weeks were either left untreated or were continued on cyclosporine medication for one to three months at a dosage of 1.5 to 7.5 mg/kg; the disease went into remission in four cases and remained controlled in the other seven, but four of the 1 1 cases relapsed during the 12 months following the treatment. The side effects observed included increased coat turnover and transient vomiting.