Spontaneous lymphocytic thyroiditis was observed at necropsy in 36 BB Wistar diabetic rats (63.2%) and in eight of their nondiabetic siblings (42.1%). The incidence of thyroiditis decreased both with age and with longer duration of diabetes. All rats with pancreatic insulitis (a manifestation of the onset of diabetes) also had thyroiditis. BB Wistar rats with insulitis had more severe lymphocytic thyroiditis, characterized by lymphocytic, plasmacytic, and macrophage infiltration of thyroid interstitium and follicles. A milder, mostly perivascular and interstitial lymphocytic thyroiditis was characteristic of lesions in rats which did not have insulitis. The histological appearance of the thyroiditis suggests that these rats may be subject to autoimmune disease at the onset of diabetes which involves sites other than just the pancreas.