Objective-To evaluate and compare circulating concentrations of islet amyloid polypep-tide (IAPP), insulin, and glucose in nondiabetic cats classifed by body condition score (BCS) and in cats with naturally occurring diabetes mellitus. Animals-109 (82 nondiabetic, 21 nonketoacidotic diabetic, and 6 ketoacidotic diabetic) cats. Procedures-Cats were examined and BCSs were assessed on a scale of 1 to 9. After food was withheld for 12 hours, blood was collected and plasma concentrations of IAPP and serum concentrations of insulin and glucose were measured. Differences in these values were evaluated among nondiabetic cats grouped according to BCS and in diabetic cats grouped as ketoacidotic or nonketoacidotic on the basis of clinicopathologic fndings. Correlations were determined among variables. Results-In nondiabetic cats, BCS was signifcantly and positively correlated with circulating IAPP and insulin concentrations. Mean plasma IAPP concentrations were signifcantly different between cats with BCSs of 5 and 7, and mean serum insulin concentrations were signifcantly different between cats with BCSs of 5 and 8. Serum glucose concentrations were not signifcantly different among nondiabetic cats. Mean IAPP concentrations were similar between nonketoacidotic diabetic cats and nondiabetic cats with BCSs of 8 or 9. Mean IAPP concentrations were signifcantly reduced in ketoacidotic diabetic cats, compared with those of nondiabetic cats with BCSs of 6 through 8 and of nonketoacidotic diabetic cats. Conclusions and Clinical Relevance-Results indicated that increased BCS (a measure of obesity) is associated with increased circulating concentrations of IAPP and insulin in nondiabetic cats.