Autoimmune reactions to parathyroid cells have been observed in human autoimmune polyendocrinopathy, but such findings have not been described in animal models of polyendocrine autoimmunity. We report here three cases of lymphocytic infiltrations in 12 parathyroid glands identified in a total of 18 thyroid glands studied in the non-obese diabetic (NOD) mouse. The majority of parathyroid-infiltrating lymphocytes possessed the helper/inducer phenotype as defined by the L3T4 monoclonal antibody. Parathyroiditis was accompanied by lymphocytic thyroiditis only on one occasion, whereas in other cases of thyroiditis, lymphocytic infiltration of the parathyroid was undetectable. We conclude that parathyroiditis in the NOD mouse is part of the wide spectrum of autoimmunity observed in this animal model of diabetes.