True thymic hyperplasia (enlarged gland composed of histologically unremarkable cortical and medullary parenchyma) and lymphoid hyperplasia (medullary lymphoid follicles in the clinical setting of autoimmunity) usually develop as independent pathologic processes. We reviewed the clinical features and gross and microscopic pathology of 2 hyperthyroid patients with features of both thymic hyperplasia and lymphoid hyperplasia. The diagnosis of thymic hyperplasia was supported by thymic weights greater than two standard deviations above the mean weight for age and histologic evidence of expanded cortical and medullary parenchyma. The diagnosis of lymphoid hyperplasia was supported by the increased number and size of medullary lymphoid follicles and the association with Graves' disease. This unusual combination results from two separate pathogenic mechanisms operating simultaneously in hyperthyroid patients. Elevated thyroid hormones directly stimulate the proliferation of thymic epithelium, producing thymic hyperplasia. The immune abnormalities underlying Graves' disease can also result in lymphoid hyperplasia of the thymus.