Hyperthyroidism is a common endocrinopathy of older cats and is associated with an increased glomerular filtration rate (gfr). Renal dysfunction is also common in older cats and may develop after they have been treated for hyperthyroidism. This paper describes the changes in the gfr of 27 hyperthyroid cats in the six months after their treatment with radioactive iodine (131I), and evaluates whether any commonly measured pretreatment parameters (serum biochemistry, systolic blood pressure, urine specific gravity) could predict a clinically significant decline in renal function. The gfr of all the cats had decreased one month after treatment, and the mean gfr was significantly lower. There was no further significant change in gfr between one and six months. The only independent variable associated with the final gfr was the pretreatment plasma glucose concentration (P=0·003).