Of 31 domestic cats with diabetes mellitus, 20 (65%) had amyloid deposits in their pancreatic islets (i.e., insular amyloid). The incidence of insular amyloidosis (p = 0.34) was not significantly different between diabetic and age-matched, non-diabetic cats from our previous study. Diabetic cats, however, had a significantly higher mean percentage of islets with amyloid (p = less than .005) and a significantly higher mean percentage of islets with abundant amyloid deposits (p = less than .005) than did non-diabetic cats. These results suggest that insular amyloidosis and diabetes are causally related and are not simply concurrent diseases associated with aging. Although the specific relationship of insular amyloidosis to the pathogenesis of diabetes mellitus was not determined, a functional islet cell abnormality probably precedes the diabetic state and the deposition of insular amyloid.