Seven cats with thyroid carcinomas that had previously undergone surgical removal of neoplastic tissue were treated with 30 mCi of radioactive iodine (131I). Six of the cats had clinical signs of hyperthyroidism; 1 did not. There were no complications associated with 131I treatment, and clinical signs resolved in all cats. Technetium scans of 4 cats made after treatment did not have evidence of isotope uptake. In the remaining 3 cats, small areas of isotope uptake, the intensity of which was equal to or less than the intensity of uptake in the salivary glands, were seen. All 7 cats became hypothyroid after treatment; 4 required L-thyroxine supplementation. One cat was alive 33 months after treatment. The other 6 cats were euthanatized because of unrelated diseases 10 to 41 months after treatment.