Cyclosporin was used, as the sole agent, to treat six cases of anal furunculosis, allowing the clinical response to the drug to be evaluated. The initial dose was 7·5 mg/kg twice a day. Following one week of treatment, whole blood cyclosporin trough levels were measured and the dose was adjusted to obtain a stable trough level of 400 to 600 ng/ml. In all dogs, the lesions were found to follow a similar pattern of resolution throughout the course of treatment. The presenting signs disappeared within one week of treatment, with a reduction in lesion size of 50 to 90 per cent. Lesions then gradually healed over the remainder of the treatment, which lasted from 10 to 20 weeks. One case required cryosurgical treatment to resolve a 2 mm diameter persistent ulcerated lesion following 18 weeks of treatment. The length of follow-up was four to 14 months (mean 7·7 months). One case developed recurrence of the lesions eight weeks post-treatment.